Remove some content_child dependency from content_common

Remove some content_child dependency from content_common

content_common does not have an explicit dependency on content_child in the GYP
files, yet depends on symbols from content_child.  This causes order-dependent
linker errors on some builds (e.g. win8_aura).  Fix this by refactoring
content::ChildThread to remove the need for that dependency, namely:

* Refactor content::ChildThread to allow users to access (and pass around) its
  content::MessageRouter member for use in IPC messaging, instead of passing
  around the ChildThread itself.
* Remove explicit check for ChildThread in content::VaapiVideoDecodeAccelerator.
* Update DEPS files accordingly.

BUG=351948
TEST=local build on desktop Linux

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=256787

[sheu, 2014-03-12 23:18:19]

This should be right up your alley.

piman@: PTAL, thanks!

[sheu, 2014-03-12 23:19:42]

Adding posciak@ since it touches VAVDA.

[piman, 2014-03-13 00:58:04]

LGTM+nit. This makes Demeter happy.

https://chromiumcodereview.appspot.com/198073003/diff/20001/content/child/chi...
File content/child/child_thread.cc (left):

https://chromiumcodereview.appspot.com/198073003/diff/20001/content/child/chi...
content/child/child_thread.cc:362: }
nit: we could keep those calls for the DCHECKs. Alternatively, maybe we could
add a DCHECK in GetRouter().

[sheu, 2014-03-13 01:50:37]

Done as piman@ suggested.

[Pawel Osciak, 2014-03-13 02:23:58]

VAVDA changes lgtm after testing

[sheu, 2014-03-13 02:24:05]

The CQ bit was checked by sheu@chromium.org

[commit-bot: I haz the power, 2014-03-13 02:24:47]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sheu@chromium.org/198073003/40001

[commit-bot: I haz the power, 2014-03-13 03:51:13]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-13 03:51:14]

Retried try job too often on android_clang_dbg for step(s) slave_steps
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=android_cl...

[sheu, 2014-03-13 03:52:44]

The CQ bit was checked by sheu@chromium.org

[commit-bot: I haz the power, 2014-03-13 03:53:34]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sheu@chromium.org/198073003/40001

[commit-bot: I haz the power, 2014-03-13 04:21:30]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-13 04:21:32]

Try jobs failed on following builders:
  tryserver.chromium on mac_chromium_rel

[sheu, 2014-03-13 06:10:40]

The CQ bit was checked by sheu@chromium.org

[commit-bot: I haz the power, 2014-03-13 06:10:43]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sheu@chromium.org/198073003/60001

[commit-bot: I haz the power, 2014-03-13 10:16:01]

Change committed as 256787