Reimplement vsync latency UMA to be based on BeginImplFrame rather than BeginMainFrame

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index ad8770af97ccd0cf399c05f16a8f41797a8dd511..6d1766a4747c06ec04ca3d2cb3f23c45fae28944 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -60319,6 +60319,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Scheduling.BeginImplFrameLatency2" units="microseconds">
+  <owner>stanisc@chromium.org</owner>
+  <summary>
+    The time from v-sync to when the main side actually starts the
+    BeginImplFrame.
+  </summary>
+</histogram>
+
 <histogram name="Scheduling.BeginMainFrameIntervalCritical"
     units="microseconds">
   <obsolete>
@@ -60657,14 +60665,6 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>How long it takes the swap ack to return after a swap.</summary>
 </histogram>
 
-<histogram name="Scheduling.VsyncToBeginMainFrameLatency2" units="microseconds">
-  <owner>stanisc@chromium.org</owner>
-  <summary>
-    The time from v-sync to when the main side actually starts the
-    BeginMainFrame.
-  </summary>
-</histogram>
-
 <histogram name="ScreenLocker.AuthenticationFailure" enum="UnlockType">
   <owner>sammiequon@chromium.org</owner>
   <summary>
@@ -115239,6 +115239,7 @@ from previous Chrome versions.
   <suffix name="Browser"/>
   <suffix name="Renderer"/>
   <affected-histogram name="Scheduling.ActivateDuration2"/>
+  <affected-histogram name="Scheduling.BeginImplFrameLatency2"/>
   <affected-histogram name="Scheduling.BeginMainFrameIntervalCritical2"/>
   <affected-histogram name="Scheduling.BeginMainFrameIntervalNotCritical2"/>
   <affected-histogram name="Scheduling.BeginMainFrameQueueDurationCritical2"/>
@@ -115253,7 +115254,6 @@ from previous Chrome versions.
   <affected-histogram name="Scheduling.PrepareTilesDuration2"/>
   <affected-histogram name="Scheduling.SwapAckWasFast"/>
   <affected-histogram name="Scheduling.SwapToAckLatency2"/>
-  <affected-histogram name="Scheduling.VsyncToBeginMainFrameLatency2"/>
 </histogram_suffixes>
 
 <histogram_suffixes name="CompositorTimingHistoryProcess2" separator="."