Revert of [Chromecast] Add explicit task_runner in CMA test

Revert of [Chromecast] Add explicit task_runner in CMA test (patchset #4 id:60001 of https://codereview.chromium.org/1267563003/)

Reason for revert:
This was actually necessary because of a bug in which I incorrectly used weak_ptr in the internal change. That problem was caught in code review so this is no longer necessary.

Original issue's description:
> [Chromecast] Use BindToCurrentLoop in CMA test
>
> Without this the callback posts tasks on whatever thread it's callback
> was originally called on, which can cause it to not respect the CMA
> pipeline's threading assumptions.
>
> Committed: https://crrev.com/9104732746ce7dd18dee6308581e8e7327bb99f1
> Cr-Commit-Position: refs/heads/master@{#341043}

TBR=erickung@google.com,kmackay@chromium.org,lcwu@google.com,halliwell@chromium.org,lcwu@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Committed: https://crrev.com/89ada2766db569000953e02db42d07eb59b05550
Cr-Commit-Position: refs/heads/master@{#341164}

[cleichner, 2015-07-30 18:43:49]

Created Revert of [Chromecast] Add explicit task_runner in CMA test

[gunsch, 2015-07-30 18:44:25]

gunsch@chromium.org changed reviewers:
+ gunsch@chromium.org

[gunsch, 2015-07-30 18:44:25]

rs lgtm

[commit-bot: I haz the power, 2015-07-30 18:44:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1254223003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1254223003/1

[commit-bot: I haz the power, 2015-07-30 18:45:44]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-30 18:46:22]

Patchset 1 (id:??) landed as
https://crrev.com/89ada2766db569000953e02db42d07eb59b05550
Cr-Commit-Position: refs/heads/master@{#341164}