viz: Record histogram of default frame rate

Useful as a base histogram for ActualIntervalForXXhz histograms added
earlier. IE can compare what the overall effect of selecting a
particular frame rate to the default.

Bug: 346732738
Change-Id: Icb2c946fb6db2c40ed9f82f020544678e743e7b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6248278
Commit-Queue: Bo Liu <boliu@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1418969}

components/viz/service/frame_sinks/root_compositor_frame_sink_impl.cc

@@ -763,11 +763,16 @@ void RootCompositorFrameSinkImpl::DisplayWillDrawAndSwap(
 
   if (external_begin_frame_source_ &&
       external_begin_frame_source_->last_begin_frame_args().IsValid() &&
-      decided_display_interval_.is_positive() &&
       base::ShouldRecordSubsampledMetric(0.001)) {
     const BeginFrameArgs& begin_frame_args =
         external_begin_frame_source_->last_begin_frame_args();
     constexpr base::TimeDelta kEpsilonTimeDelta = base::Milliseconds(0.5);
+    if (decided_display_interval_.is_zero()) {
+      base::UmaHistogramCustomTimes(
+          "Viz.FrameIntervalDecider.ActualIntervalDefault",
+          begin_frame_args.interval, base::Milliseconds(0),
+          base::Milliseconds(500), 50);
+    }
     if ((decided_display_interval_ - base::Hertz(30)).magnitude() <
         kEpsilonTimeDelta) {
       base::UmaHistogramCustomTimes(

tools/metrics/histograms/metadata/gpu/histograms.xml

@@ -1862,6 +1862,23 @@ chromium-metrics-reviews@google.com.
   </token>
 </histogram>
 
+<histogram name="Viz.FrameIntervalDecider.ActualIntervalDefault" units="ms"
+    expires_after="M143">
+  <owner>boliu@chromium.org</owner>
+  <owner>chrome-gpu-metric-alerts@chromium.org</owner>
+  <summary>
+    Record the actual frame rate when FrameIntervalDecider decides system should
+    use its default behavior. Useful as reference histogram for
+    ActualIntervalForXXhz histograms, to see the effect of FrameIntervalDecider
+    deciding on a particular frame rate. Note however it's not a perfect
+    reference since the conditions leading to use system default is not
+    independent from conditions leading a particular frame rate being selected.
+    Recorded every viz frame, sub-sampled with probability 0.001. Note this
+    requires FrameIntervalDecider to be enabled as well as using an
+    ExternalBeginFrameSource.
+  </summary>
+</histogram>
+
 <histogram name="Viz.FrameIntervalDecider.ActualIntervalFor{DecidedHz}hz"
     units="ms" expires_after="M143">
   <owner>boliu@chromium.org</owner>