Measure usage of the prefixed encrypted media APIs

Source/core/frame/UseCounter.h

Index: Source/core/frame/UseCounter.h
diff --git a/Source/core/frame/UseCounter.h b/Source/core/frame/UseCounter.h
index 64cd844654c1eee630083dc2c6931e5d1344d23a..7e1e67c5129122cc1dcc7946eaf7de49df299711 100644
--- a/Source/core/frame/UseCounter.h
+++ b/Source/core/frame/UseCounter.h
@@ -285,6 +285,7 @@ public:
         ElementSetPrefix, // Element.prefix is readonly in DOM4.
         CSSStyleDeclarationGetPropertyCSSValue,
         SVGElementGetPresentationAttribute,
+        PrefixedMediaCancelKeyRequest,

[ddorwin, 2014/01/16 18:14:30]

We don't really need this. Nobody is using it. We don't even really need the
AddKey one since GenerateKeyRequest must be called first.

(canPlayType's keySystem parameter and webkitGenerateKeyRequest() are the
entrypoints to prefixed EME.)

[philipj_slow, 2014/01/16 18:46:20]

I see what you're saying about one call being meaningless without the other, but
the fullscreen API is similar and has counters for all functions... it helps to
remove any doubt in an intent to deprecate and remove. Honestly I hadn't thought
about it before, but for simplicity I'll go with this, unless you object.

         // Add new features immediately above this line. Don't change assigned
         // numbers of each items, and don't reuse unused slots.
         NumberOfFeatures, // This enum value must be last.