Add BeginFrameLatency UMA

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 3670e9d20f50d763404c55cd8b62b1dead75b309..8e1914e305e31d2dca4ce77772ab13935cba980e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -60633,6 +60633,14 @@ 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>
@@ -115142,6 +115150,7 @@ 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="."