Avoid running DocumentLoader::StartLoadingResponse during initial commit

DocumentLoader::StartLoadingResponse body can take multiple ms and post
a bunch of tasks when initializing the renderer with the initial empty
document. This is not necessary, since the document doesn't have
anything to load. This change adds a feature to avoid doing this. If
the results look promising, I may look into other renderer init
improvements.

Bug: 332706093
Change-Id: I503f4851ebe722e50a513886512b323e856f481a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5502925
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1299623}

android_webview/java/src/org/chromium/android_webview/common/ProductionSupportedFlagList.java

@@ -995,6 +995,7 @@ public final class ProductionSupportedFlagList {
         Flag.baseFeature(
                 CcFeatures.METRICS_TRACING_CALCULATION_REDUCTION,
                 "Reduces Renderer event latency attribution to only during tracing."),
+        Flag.baseFeature(BlinkFeatures.STREAMLINE_RENDERER_INIT),
         // Add new commandline switches and features above. The final entry should have a
         // trailing comma for cleaner diffs.
     };

testing/variations/fieldtrial_testing_config.json

@@ -19261,6 +19261,27 @@
             ]
         }
     ],
+    "StreamlineRendererInit": [
+        {
+            "platforms": [
+                "android",
+                "android_webview",
+                "chromeos",
+                "chromeos_lacros",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "StreamlineRendererInit"
+                    ]
+                }
+            ]
+        }
+    ],
     "StructuredMetricsExternalMetrics": [
         {
             "platforms": [

third_party/blink/common/features.cc

@@ -2296,6 +2296,11 @@ BASE_FEATURE(kStopInBackground,
 #endif
 );
 
+// Reduces the work done during renderer initialization.
+BASE_FEATURE(kStreamlineRendererInit,
+             "StreamlineRendererInit",
+             base::FEATURE_DISABLED_BY_DEFAULT);
+
 BASE_FEATURE(kStylusPointerAdjustment,
              "StylusPointerAdjustment",
              base::FEATURE_ENABLED_BY_DEFAULT);

third_party/blink/public/common/features.h

@@ -1518,6 +1518,8 @@ BLINK_COMMON_EXPORT extern const base::FeatureParam<bool>
 
 BLINK_COMMON_EXPORT BASE_DECLARE_FEATURE(kStopInBackground);
 
+BLINK_COMMON_EXPORT BASE_DECLARE_FEATURE(kStreamlineRendererInit);
+
 // Stylus gestures for editable web content.
 BLINK_COMMON_EXPORT BASE_DECLARE_FEATURE(kStylusRichGestures);
 // Apply touch adjustment for stylus pointer events. This feature allows

third_party/blink/renderer/core/loader/document_loader.cc

@@ -97,8 +97,11 @@
 #include "third_party/blink/renderer/core/frame/navigator.h"
 #include "third_party/blink/renderer/core/frame/settings.h"
 #include "third_party/blink/renderer/core/frame/web_feature.h"
+#include "third_party/blink/renderer/core/html/html_body_element.h"
 #include "third_party/blink/renderer/core/html/html_document.h"
 #include "third_party/blink/renderer/core/html/html_frame_owner_element.h"
+#include "third_party/blink/renderer/core/html/html_head_element.h"
+#include "third_party/blink/renderer/core/html/html_html_element.h"
 #include "third_party/blink/renderer/core/html/html_object_element.h"
 #include "third_party/blink/renderer/core/html/parser/html_parser_idioms.h"
 #include "third_party/blink/renderer/core/html/parser/text_resource_decoder_builder.h"
@@ -1932,6 +1935,23 @@ void DocumentLoader::StartLoadingResponse() {
   if (!frame_)
     return;
 
+  // TODO(crbug.com/332706093): See if this optimization can be enabled for
+  // non-main frames after fixing failing tests.
+  if (base::FeatureList::IsEnabled(features::kStreamlineRendererInit) &&
+      frame_->IsMainFrame() && loading_url_as_empty_document_ &&
+      commit_reason_ == CommitReason::kInitialization) {
+    // We know this is an empty document, so explicitly set empty content
+    // without going through the parser, which has a lot of overhead.
+    Document* document = frame_->GetDocument();
+    auto* html = MakeGarbageCollected<HTMLHtmlElement>(*document);
+    html->AppendChild(MakeGarbageCollected<HTMLHeadElement>(*document));
+    document->AppendChild(html);
+    html->AppendChild(MakeGarbageCollected<HTMLBodyElement>(*document));
+
+    FinishedLoading(base::TimeTicks::Now());
+    return;
+  }
+
   CHECK_GE(state_, kCommitted);
 
   CreateParserPostCommit();