Forbid ImageDownloaderImpl from accessing RenderThread instance after message loop shutdown.

Forbid ImageDownloaderImpl from accessing RenderThread instance after message loop shutdown.

This CL fixes the crash caused by following possible scenario:
- the message loop is shutdown.
- during its destructor it calls connection error on ImageDownloaderImpl.
- the strong binding of ImageDownloaderImpl causes it to be destructed.
- the destructor calls RenderThread::Get()->RemoveObserver(this),
  which is not valid any more.

BUG=640999
TBR=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2286603002
Cr-Commit-Position: refs/heads/master@{#414985}
(cherry picked from commit d2ac649bc631233f4355860a011d3d30f985bb67)

Committed: https://chromium.googlesource.com/chromium/src/+/a64d2294bbe77cf839715bfefa407d61147e79d6

[vabr (Chromium), 2016-09-13 10:19:07]

Description was changed from

==========
Forbid ImageDownloaderImpl from accessing RenderThread instance after message
loop shutdown.

This CL fixes the crash caused by following possible scenario:
- the message loop is shutdown.
- during its destructor it calls connection error on ImageDownloaderImpl.
- the strong binding of ImageDownloaderImpl causes it to be destructed.
- the destructor calls RenderThread::Get()->RemoveObserver(this),
  which is not valid any more.

BUG=640999
TBR=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2286603002
Cr-Commit-Position: refs/heads/master@{#414985}
(cherry picked from commit d2ac649bc631233f4355860a011d3d30f985bb67)
==========

to

==========
Forbid ImageDownloaderImpl from accessing RenderThread instance after message
loop shutdown.

This CL fixes the crash caused by following possible scenario:
- the message loop is shutdown.
- during its destructor it calls connection error on ImageDownloaderImpl.
- the strong binding of ImageDownloaderImpl causes it to be destructed.
- the destructor calls RenderThread::Get()->RemoveObserver(this),
  which is not valid any more.

BUG=640999
TBR=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2286603002
Cr-Commit-Position: refs/heads/master@{#414985}
(cherry picked from commit d2ac649bc631233f4355860a011d3d30f985bb67)

Committed:
https://chromium.googlesource.com/chromium/src/+/a64d2294bbe77cf839715bfefa40...
==========

[vabr (Chromium), 2016-09-13 10:19:09]

Committed patchset #1 (id:1) manually as
a64d2294bbe77cf839715bfefa407d61147e79d6.