Add UMA to track software fallback in VTVideoDecodeAccelerator.

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 f46a8b87c2a38de7735ce4a4eeb88030552d6451..7cb534f37205800d5203ee4345eb8ce0d5e159a4 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12747,6 +12747,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>Visible width of HTML5 video.</summary>
 </histogram>
 
+<histogram name="Media.VTVDA.HardwareAccelerated"
+    enum="BooleanHardwareAccelerated">
+  <owner>sandersd@chromium.org</owner>
+  <summary>
+    Whether a VTDecompressionSession is internally using hardware accelerated
+    decoding.
+  </summary>
+</histogram>
+
 <histogram name="Media.YouTube.DelayedAndDroppedFramesPer5Sec"
     units="frames/5s">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
@@ -41808,6 +41817,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="1" label="Handshake confirmed"/>
 </enum>
 
+<enum name="BooleanHardwareAccelerated" type="int">
+  <int value="0" label="Not hardware accelerated"/>
+  <int value="1" label="Hardware accelerated"/>
+</enum>
+
 <enum name="BooleanHasCrc" type="int">
   <int value="0" label="No CRC"/>
   <int value="1" label="Has CRC"/>