Collect host of Web Component usage to send to RAPPOR

Source/core/frame/UseByOriginCounter.h

Index: Source/core/frame/UseByOriginCounter.h
diff --git a/Source/core/frame/UseByOriginCounter.h b/Source/core/frame/UseByOriginCounter.h
new file mode 100644
index 0000000000000000000000000000000000000000..b552d611f2a1488307e577bb86087bfb9ff99d97
--- /dev/null
+++ b/Source/core/frame/UseByOriginCounter.h
@@ -0,0 +1,59 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UseByOriginCounter_h
+#define UseByOriginCounter_h
+
+#include "platform/heap/Handle.h"
+#include "wtf/Vector.h"
+#include "wtf/text/WTFString.h"
+
+namespace blink {
+
+class Document;
+class ScriptState;
+
+class UseByOriginCounter {

[tkent, 2015/03/16 00:30:19]

I feel this is not a counter.  Is this origin-using-features-reporter?

[kojii, 2015/03/16 03:52:51]

Done.

+    DISALLOW_ALLOCATION();
+public:
+    ~UseByOriginCounter();
+
+    enum class Feature {
+        ElementCreateShadowRoot,
+        DocumentRegisterElement,
+
+        NumberOfFeatures // This must be the last item.
+    };
+
+    static void count(const ScriptState*, Document&, Feature);
+
+    void documentDetached(Document&);
+    void updateMeasurementsAndClear();
+
+    class Value {
+    public:
+        Value();
+
+        bool isEmpty() const { return !m_countBits; }
+        void clear() { m_countBits = 0; }
+
+        void count(Feature);
+        bool get(Feature feature) const { return m_countBits & (1 << (unsigned)feature); }

[tkent, 2015/03/16 00:30:19]

We don't use C-style cast.  (unsigned) -> static_cast<unsigned>()

[kojii, 2015/03/16 03:52:51]

Done.

+
+        void aggregate(Value);
+        void updateMeasurements(const String& origin);
+
+    private:
+        unsigned m_countBits : (unsigned)Feature::NumberOfFeatures;

[tkent, 2015/03/16 00:30:19]

Ditto.

Also, do you think the number of features won't grow over the size of an
integer?  UseCounter had similar problem, and we switched to BitVector.

[kojii, 2015/03/16 03:52:51]

Done.
Looks like it's unknown. Heard an idea to turn all features in UseCounter to use
RAPPOR. But then that'd mean we'll upload almost every domain names users visit,
and we weren't sure we have enough justifications to do so.

So my strategy here was to have the minimum code to support the requirements for
now, while making it close to UseCounter by seeing the possibility to move to
BitVector or UseCounter::CountBits, or even merge this class into UseCounter.
Does this look ok?

+    };
+
+    DEFINE_INLINE_TRACE() { visitor->trace(m_originAndValues); }

[tkent, 2015/03/16 00:30:19]

Oilpan tracing is not necessary because this class isn't GarbageCollected.

[kojii, 2015/03/16 03:52:51]

Done, I thought we need because it has String, sorry for misunderstanding.

+
+private:
+    WillBeHeapVector<std::pair<String, UseByOriginCounter::Value>, 1> m_originAndValues;

[tkent, 2015/03/16 00:30:19]

Ditto.
should be Vector<std::pair<String, UseByOriginCounter::Value>, 1>.

[kojii, 2015/03/16 03:52:51]

Done.

+};
+
+} // namespace blink
+
+#endif // UseByOriginCounter_h