Makes WebThreadSupportingGC constructible for an existing thread

Source/platform/WebThreadSupportingGC.h

 // Copyright 2014 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 WebThreadSupportingGC_h
 #define WebThreadSupportingGC_h
 
 #include "platform/heap/glue/MessageLoopInterruptor.h"
 #include "platform/heap/glue/PendingGCRunner.h"
 #include "public/platform/Platform.h"
 #include "public/platform/WebThread.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace blink {
 
 // WebThreadSupportingGC wraps a WebThread and adds support for attaching
 // to and detaching from the Blink GC infrastructure. The initialize method
 // must be called during initialization on the WebThread and before the
 // thread allocates any objects managed by the Blink GC. The shutdown
 // method must be called on the WebThread during shutdown when the thread
 // no longer needs to access objects managed by the Blink GC.
+//
+// WebThreadSupportingGC usually internally creates and owns WebThread unless
+// an existing WebThread is given via createForThread.
 class PLATFORM_EXPORT WebThreadSupportingGC final {
     WTF_MAKE_NONCOPYABLE(WebThreadSupportingGC);
 public:
-    static PassOwnPtr<WebThreadSupportingGC> create(const char*);
+    static PassOwnPtr<WebThreadSupportingGC> create(const char* name);
+    static PassOwnPtr<WebThreadSupportingGC> createForThread(WebThread*);
     ~WebThreadSupportingGC();
 
     void postTask(const WebTraceLocation& location, WebThread::Task* task)
     {
         m_thread->postTask(location, task);
     }
 
     void postDelayedTask(const WebTraceLocation& location, WebThread::Task* task, long long delayMs)
     {
         m_thread->postDelayedTask(location, task, delayMs);
@@ skipping 17 matching lines @@
     void initialize();
     void shutdown();
 
     WebThread& platformThread() const
     {
         ASSERT(m_thread);
         return *m_thread;
     }
 
 private:
-    explicit WebThreadSupportingGC(const char*);
+    WebThreadSupportingGC(const char* name, WebThread*);
 
     OwnPtr<PendingGCRunner> m_pendingGCRunner;
-    OwnPtr<WebThread> m_thread;
+
+    // m_thread is guaranteed to be non-null after this instance is constructed.
+    // m_owningThread is non-null unless this instance is constructed for an
+    // existing thread via createForThread().
+    WebThread* m_thread = nullptr;
+    OwnPtr<WebThread> m_owningThread;
 };
 
 }
 
 #endif