web-threads: Create a single instance of WebThread for each blink thread.

content/child/blink_platform_impl.h

Index: content/child/blink_platform_impl.h
diff --git a/content/child/blink_platform_impl.h b/content/child/blink_platform_impl.h
index 9445ea662f59253cfb5c5d0d0b709123e49b032b..b7230a9fb894c3d83a9800e67fa58df04cc36623 100644
--- a/content/child/blink_platform_impl.h
+++ b/content/child/blink_platform_impl.h
@@ -176,14 +176,13 @@ class CONTENT_EXPORT BlinkPlatformImpl
   virtual int domEnumFromCodeString(const blink::WebString& codeString);
 
  private:
-  static void DestroyCurrentThread(void*);
-
   void DoTimeout() {
     if (shared_timer_func_ && !shared_timer_suspended_)
       shared_timer_func_();
   }
 
   void InternalInit();
+  void UpdateWebThreadTLS(blink::WebThread* thread);
 
   scoped_refptr<base::SingleThreadTaskRunner> MainTaskRunnerForCurrentThread();