Measure usage of the prefixed encrypted media APIs

Source/core/frame/UseCounter.h

 /*
  * Copyright (C) 2012 Google, Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 267 matching lines @@
         ElementPrefixedMatchesSelector,
         DOMImplementationCreateCSSStyleSheet,
         CSSStyleSheetRules,
         CSSStyleSheetAddRule,
         CSSStyleSheetRemoveRule,
         InitMessageEvent,
         PrefixedInitMessageEvent,
         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.
     };
 
     // "count" sets the bit for this feature to 1. Repeated calls are ignored.
     static void count(const Document&, Feature);
     static void count(const ExecutionContext*, Feature);
     void count(CSSParserContext, CSSPropertyID);
     void count(Feature);
@@ skipping 35 matching lines @@
 
     void updateMeasurements();
 
     OwnPtr<BitVector> m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace WebCore
 
 #endif // UseCounter_h