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 a21c7fd4311b1cce5b368145325ac1d86dc7446a..f4957396605b51eb98c747ed96d9bdbb8998f6bc 100644
--- a/content/child/blink_platform_impl.h
+++ b/content/child/blink_platform_impl.h
@@ -168,14 +168,13 @@ 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();