Add new UseCounter metric

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 f42a496e4a02eb2725ae0f773c04c99303ac0ebc..fb1d9619255ff73a50db350583567bd0ddd783c2 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.h
+++ b/third_party/WebKit/Source/core/frame/UseCounter.h
@@ -59,12 +59,11 @@ class CORE_EXPORT UseCounter {
     WTF_MAKE_NONCOPYABLE(UseCounter);
 public:
     UseCounter();
-    ~UseCounter();
 
     enum Feature {
         // Do not change assigned numbers of existing items: add new features
         // to the end of the list.
-        PageDestruction = 0,
+        OBSOLETE_PageDestruction = 0,
         PrefixedIndexedDB = 3,
         WorkerStart = 4,
         SharedWorkerStart = 5,
@@ -1338,11 +1337,26 @@ public:
     bool hasRecordedMeasurement(Feature) const;
 
 protected:
-    friend class UseCounterTest;
+    // Encapsulates the work to preserve the old "FeatureObserver" histogram with original semantics
+    // TODO(rbyers): remove this - http://crbug.com/597963
+    class LegacyCounter {

[Nico, 2016/09/13 14:16:47]

this needs a CORE_EXPORT, else release component builds won't link (at least
with clang-cl):

FAILED: webkit_unit_tests.exe webkit_unit_tests.exe.pdb 
E:/b/depot_tools/python276_bin/python.exe
../../build/toolchain/win/tool_wrapper.py link-wrapper environment.x64 False
link.exe /nologo /OUT:./webkit_unit_tests.exe /PDB:./webkit_unit_tests.exe.pdb
@./webkit_unit_tests.exe.rsp
UseCounterTest.obj : error LNK2019: unresolved external symbol "public: __cdecl
blink::UseCounter::LegacyCounter::~LegacyCounter(void)"
(??1LegacyCounter@UseCounter@blink@@QEAA@XZ) referenced in function "private:
virtual void __cdecl
blink::UseCounterTest_RecordingFeatures_Test::TestBody(void)"
(?TestBody@UseCounterTest_RecordingFeatures_Test@blink@@EEAAXXZ)
./webkit_unit_tests.exe : fatal error LNK1120: 1 unresolved externals

https://build.chromium.org/p/chromium.fyi/builders/CrWinClang%28shared%29/bui...

+    public:
+        LegacyCounter();
+        ~LegacyCounter();
+        void countFeature(Feature);
+        void countCSS(CSSPropertyID);
+        void updateMeasurements();
+    private:
+        // Tracks what features/properties need to be reported to the legacy histograms.
+        BitVector m_featureBits;
+        BitVector m_CSSBits;
+    } m_legacyCounter;
+
     unsigned m_muteCount;
 
-    BitVector m_featureBits;
-    BitVector m_CSSFeatureBits;
+    // Track what features/properties have been reported to the (non-legacy) histograms.
+    BitVector m_featuresRecorded;
+    BitVector m_CSSRecorded;
 };
 
 } // namespace blink