BeginMainSentToStarted is added to histograms.xml

The metric was added in an update before to track the time begin main
frame sent to start:
https://chromium-review.googlesource.com/c/chromium/src/+/2013461

Bug: chromium:976030
Change-Id: Ib575aef2feb3ed94c6e2cff2b83f2ecc4543c347
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028109Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736506}

tools/metrics/histograms/histograms.xml

@@ -178292,6 +178292,9 @@ regressions. -->
   <suffix name="SendBeginMainFrameToCommit.Animate"
       label="The time portion of SendBeginMainFrameToCommit spent on
              animations."/>
+  <suffix name="SendBeginMainFrameToCommit.BeginMainSentToStarted"
+      label="The time portion of SendBeginMainFrameToCommit spent before
+             starting main thread work."/>
   <suffix name="SendBeginMainFrameToCommit.Composite"
       label="The time portion of SendBeginMainFrameToCommit spent on
              compositing."/>