Rename FeatureObserver to UseCounter to make clear that all it stores are usage counts.

Source/WebCore/page/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
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY GOOGLE INC. ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE COMPUTER, INC. OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef FeatureObserver_h
-#define FeatureObserver_h
+#ifndef UseCounter_h
+#define UseCounter_h
 
 #include <wtf/BitVector.h>
 #include <wtf/Noncopyable.h>
 #include <wtf/OwnPtr.h>
 #include <wtf/PassOwnPtr.h>
 
 namespace WebCore {
 
 class DOMWindow;
 class Document;
 
-class FeatureObserver {
-    WTF_MAKE_NONCOPYABLE(FeatureObserver);
+// UseCounter is used for counting the number of times features of
+// Blink are used on real web pages.  The counts are submitted

[darin (slow to review), 2013/04/16 21:28:51]

nit: You might consider revisions to this wording as this data could also be
treated very differently by other embedders of Blink / Chromium.  Maybe, "For
instance, in Google Chrome blah blah blah..."

+// anonymously through the Histogram recording system in Chrome
+// for users who opt-in to "Usage Statistics" submission
+// during their install of Google Chrome:
+// http://www.google.com/chrome/intl/en/privacy.html
+// These numbers are used to help us know how commonly features of
+// Blink are used, and thus when it's safe to remove or change them.
+class UseCounter {
+    WTF_MAKE_NONCOPYABLE(UseCounter);
 public:
-    FeatureObserver();
-    ~FeatureObserver();
+    UseCounter();
+    ~UseCounter();
 
     enum Feature {
         PageDestruction,
         LegacyNotifications,
         MultipartMainResource,
         PrefixedIndexedDB,
         WorkerStart,
         SharedWorkerStart,
         LegacyWebAudio,
         WebAudioStart,
@@ skipping 50 matching lines @@
         XFrameOptions,
         XFrameOptionsSameOrigin,
         XFrameOptionsSameOriginWithBadAncestorChain,
         DeprecatedFlexboxWebContent,
         DeprecatedFlexboxChrome,
         DeprecatedFlexboxChromeExtension,
         // Add new features above this line. Don't change assigned numbers of each items.
         NumberOfFeatures, // This enum value must be last.
     };
 
+    // "observe" sets the bit for this feature to 1. Repeated calls are ignored.
     static void observe(Document*, Feature);
     static void observe(DOMWindow*, Feature);
     void didCommitLoad();
 
-    const BitVector* accumulatedFeatureBits() const { return m_featureBits.get(); }
-
 private:
     void didObserve(Feature feature)
     {
         ASSERT(feature != PageDestruction); // PageDestruction is reserved as a scaling factor.
         ASSERT(feature < NumberOfFeatures);
-        if (!m_featureBits) {
-            m_featureBits = adoptPtr(new BitVector(NumberOfFeatures));
-            m_featureBits->clearAll();
+        if (!m_countBits) {
+            m_countBits = adoptPtr(new BitVector(NumberOfFeatures));
+            m_countBits->clearAll();
         }
-        m_featureBits->quickSet(feature);
+        m_countBits->quickSet(feature);
     }
 
     void updateMeasurements();
 
-    OwnPtr<BitVector> m_featureBits;
+    OwnPtr<BitVector> m_countBits;
 };
 
 } // namespace WebCore
-    
-#endif // FeatureObserver_h
+
+#endif // UseCounter_h