Will resume work on Android and other platforms where the DRM resources might have been reclaimed by the system?

Today when we suspend we only destroy the media Renderer. The media element will stay alive and keep holding a ref to the MediaKeys/CDM.

This should block next dev release.

 Issue 595578  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7655364f9ae672fdbd05e5751006387207b0bf51

commit 7655364f9ae672fdbd05e5751006387207b0bf51
Author: sandersd <sandersd@chromium.org>
Date: Fri Mar 18 22:14:35 2016

Restore CDM context on Resume().

Also converts all uses of |weak_factory_| to use a |weak_this_|, as GetWeakPtr() is not threadsafe.

BUG= 595898 

Review URL: https://codereview.chromium.org/1815013002

Cr-Commit-Position: refs/heads/master@{#382109}

[modify] https://crrev.com/7655364f9ae672fdbd05e5751006387207b0bf51/media/base/pipeline_impl.cc
[modify] https://crrev.com/7655364f9ae672fdbd05e5751006387207b0bf51/media/base/pipeline_impl.h

Please remove the blocker label if its resolved.