The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/362d7c2729d0526102e5faf715091ac887b22746

commit 362d7c2729d0526102e5faf715091ac887b22746
Author: Xi Han <hanxi@google.com>
Date: Fri Oct 05 19:27:58 2018

Move |media_thread| from CastBrowserMainParts to CastContentBrowserClient.

In the new startup code, ServiceManagerContext (SMC) will be created before
the BrowserMainRunner is created. In the constructor of SMC, the
GetMediaTaskRunner() accesses the TaskRunner of a media thread created by
CastBrowserMainParts which will be created after the BrowserMainRunner. So
we make the CastContentBrowserClient own the media thread and create it upon
request.

Bug:  887638 
Change-Id: I8986f5b00600760afc8bf00bac140f459c7145d1
Reviewed-on: https://chromium-review.googlesource.com/c/1258204
Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597239}
[modify] https://crrev.com/362d7c2729d0526102e5faf715091ac887b22746/chromecast/browser/cast_browser_main_parts.cc
[modify] https://crrev.com/362d7c2729d0526102e5faf715091ac887b22746/chromecast/browser/cast_browser_main_parts.h
[modify] https://crrev.com/362d7c2729d0526102e5faf715091ac887b22746/chromecast/browser/cast_browser_main_parts_simple.cc
[modify] https://crrev.com/362d7c2729d0526102e5faf715091ac887b22746/chromecast/browser/cast_content_browser_client.cc
[modify] https://crrev.com/362d7c2729d0526102e5faf715091ac887b22746/chromecast/browser/cast_content_browser_client.h