Add YV12HD to VideoPixelFormat histogram enum.

This value corresponds to media::VideoFrame::YV12HD which was added in a previous CL.

BUG=None

Review URL: https://codereview.chromium.org/948633002

Cr-Commit-Position: refs/heads/master@{#317714}

tools/metrics/histograms/histograms.xml

@@ -60344,6 +60344,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="8" label="NV12"/>
   <int value="9" label="YV24"/>
   <int value="10" label="ARGB"/>
+  <int value="11" label="YV12HD"/>
 </enum>
 
 <enum name="VideoPlayerCastAPIExtensionStatus" type="int">