Make sure FrameScheduler is cleared when LocalFrame::detach() finishes

components/scheduler/child/single_thread_idle_task_runner.cc

 // 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.
 
 #include "components/scheduler/child/single_thread_idle_task_runner.h"
 
 #include "base/location.h"
 #include "base/trace_event/blame_context.h"
 #include "base/trace_event/trace_event.h"
 
@@ skipping 66 matching lines @@
   if (blame_context_)
     blame_context_->Enter();
   idle_task.Run(deadline);
   if (blame_context_)
     blame_context_->Leave();
   delegate_->DidProcessIdleTask();
 }
 
 void SingleThreadIdleTaskRunner::SetBlameContext(
     base::trace_event::BlameContext* blame_context) {
-  blame_context_ = blame_context;
+  blame_context_ = blame_context ? blame_context->weak_ptr() : nullptr;

[Primiano Tucci (use gerrit), 2016/06/10 07:50:35]

how do you know that here you are on the right thread (the one where
blame_context lives) so invoking weak_ptr() is safe?

[Xiaocheng, 2016/06/10 08:07:08]

Are you implying any potentially risky design?

In my understanding, it's currently safe. The only call site of
SingleThreadIdleTaskRunner::SetBlameContext() is in
RendererBlinkPlatformImpl::ctor(), which can only be run in the main thread. The
only BlameContext that can be passed to this function is the singleton
RendererBlinkPlatformImpl::top_level_blame_context_, which also live in the main
thread.

I can also add a DCHECK(thread_checker_.CalledOnValidThread()) in
BlameContext::weak_ptr() to make sure that it's called on the correct thread.

 }
 
 }  // namespace scheduler