Not for review - Rebase of crrev.com/62833003

Source/platform/UIWebThreadSupportingGC.h

+#ifndef UIWEBTHREADSUPPORTINGGC
+#define UIWEBTHREADSUPPORTINGGC
+
+#include "WebThreadSupportingGC.h"
+#include "wtf/HashMap.h"
+
+namespace blink {
+
+class PLATFORM_EXPORT UIWebThreadSupportingGC : public WebThreadSupportingGC {
+    WTF_MAKE_NONCOPYABLE(UIWebThreadSupportingGC);
+  public:
+    static PassOwnPtr<UIWebThreadSupportingGC> create(const char* name);
+    static UIWebThreadSupportingGC* get(WebThread* t);
+
+    virtual void attachGC() override;
+    virtual void detachGC() override;
+
+  private:
+    // should never create 2 UIWebThreadSupportingGCs pointing to the same thread
+    UIWebThreadSupportingGC(const char* name);
+    UIWebThreadSupportingGC(WebThread* t);
+
+    typedef HashMap<WebThread*, UIWebThreadSupportingGC*> ThreadMap;
+    static ThreadMap* m_threadMap;
+
+    int m_attachedThreads;
+};
+
+}
+
+#endif /* ifndef UIWEBTHREADSUPPORTINGGC */