HTMLVideoElement: add metrics for poster attribute usage.

third_party/WebKit/Source/core/frame/UseCounter.h

Index: third_party/WebKit/Source/core/frame/UseCounter.h
diff --git a/third_party/WebKit/Source/core/frame/UseCounter.h b/third_party/WebKit/Source/core/frame/UseCounter.h
index aa2da2613a19843fa454ae979e11a3353119e371..b7d52907d26f0320a9561ae0aa6c109f1aef44ed 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.h
+++ b/third_party/WebKit/Source/core/frame/UseCounter.h
@@ -1462,6 +1462,8 @@ class CORE_EXPORT UseCounter {
     V8FileReaderSync_Constructor = 1812,
     UncancellableTouchEventPreventDefaulted = 1813,
     UncancellableTouchEventDueToMainThreadResponsivenessPreventDefaulted = 1814,
+    V8HTMLVideoElement_Poster_AttributeGetter = 1815,
+    V8HTMLVideoElement_Poster_AttributeSetter = 1816,
 
     // Add new features immediately above this line. Don't change assigned
     // numbers of any item, and don't reuse removed slots.