cc: Use correct message loop proxy in BlockingTaskRunner

cc/trees/blocking_task_runner.h

Index: cc/trees/blocking_task_runner.h
diff --git a/cc/trees/blocking_task_runner.h b/cc/trees/blocking_task_runner.h
index 8388a881e607e6bbcf11e4c30b9d659f1dd54909..0bc23b34c1732a440c77b250fcfe3cf060c33998 100644
--- a/cc/trees/blocking_task_runner.h
+++ b/cc/trees/blocking_task_runner.h
@@ -72,6 +72,11 @@ class CC_EXPORT BlockingTaskRunner
   bool PostTask(const tracked_objects::Location& from_here,
                 const base::Closure& task);
 
+  // Forward the tasks to a specific task runner instead of the

[danakj, 2014/08/26 15:59:00]

WDYT of passing this to current() and DCHECKing that it matches the existing
BlockingTaskRunner if one already exists?

Would the changes to Proxy no longer be needed?

[Sami, 2014/08/26 16:19:17]

The places[1] that call BlockingTaskRunner::current() don't have an easy way to
access the main_task_runner_ from Proxy :( Or did you mean something like the
runner only needs to be passed in the first time this is called and after that
it sticks?

[1] DelegatedFrameResourceCollection, DelegatedRendererLayer and TextureLayer

+  // MessageLoopProxy which was current when this task runner was constructed.
+  // Must not be called while capturing tasks.
+  void SetTaskRunner(scoped_refptr<base::SingleThreadTaskRunner> task_runner);
+
  private:
   friend class base::RefCountedThreadSafe<BlockingTaskRunner>;