Add win_compiler_timing switch

Change-Id: I32097bc02654e74251e309dbd550e916203215a9
Reviewed-on: https://chromium-review.googlesource.com/734296
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511525}

build/config/win/BUILD.gn

@@ -19,6 +19,9 @@ declare_args() {
 
   # Turn this on to have the linker output extra timing information.
   win_linker_timing = false
+
+  # Turn this on to have the compiler output extra timing information.
+  win_compiler_timing = false
 }
 
 # This is included by reference in the //build/config/compiler config that
@@ -120,6 +123,21 @@ config("compiler") {
     # the source file is a no-op.
     "/ignore:4221",
   ]
+
+  if (win_compiler_timing) {
+    if (is_clang) {
+      cflags += [
+        "-Xclang",
+        "-ftime-report",
+      ]
+    } else {
+      cflags += [
+        # "Documented" here:
+        # http://aras-p.info/blog/2017/10/23/Best-unknown-MSVC-flag-d2cgsummary/
+        "/d2cgsummary",
+      ]
+    }
+  }
 }
 
 config("vs_code_analysis") {

docs/windows_build_instructions.md

@@ -209,8 +209,6 @@ support incremental linking for more targets. Note that if you set this but
 don't' set enable_nacl = false then build times may get worse.
 * `remove_webcore_debug_symbols = true` - turn off source-level debugging for
 blink to reduce build times, appropriate if you don't plan to debug blink.
-* `win_linker_timing = true` - this should not generally be set but can be
-helpful when trying to understand build times or incremental linking failures.
 
 In addition, Google employees should consider using goma, a distributed
 compilation system. Detailed information is available internally but the