Fix a crash that MediaPlayerAndroid weak_ptr is incorrectly used across thread

Fix a crash that MediaPlayerAndroid weak_ptr is incorrectly used across thread

For MediaPlayerListener, the callbacks can happen on a non-UI thread.
Weak_ptr is not thread safe, so we shouldn't bind the weak_ptr in those callbacks.
This change creates a threadsafe proxy class to bridge all the callbacks.

BUG=453581, 447368, 447367
Committed: https://crrev.com/2e9885788692f69203a98fb42347428ef553d640
Cr-Commit-Position: refs/heads/master@{#314241}

[qinmin, 2015-02-02 19:52:52]

qinmin@chromium.org changed reviewers:
+ xhwang@chromium.org

[qinmin, 2015-02-02 19:52:52]

PTAL

[xhwang, 2015-02-02 20:53:54]

lgtm

[qinmin, 2015-02-02 21:13:59]

The CQ bit was checked by qinmin@chromium.org

[commit-bot: I haz the power, 2015-02-02 21:14:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/899473002/1

[commit-bot: I haz the power, 2015-02-03 01:56:54]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-03 01:57:38]

Patchset 1 (id:??) landed as
https://crrev.com/2e9885788692f69203a98fb42347428ef553d640
Cr-Commit-Position: refs/heads/master@{#314241}

[qinmin, 2015-02-09 18:47:22]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/879273006/ by qinmin@chromium.org.

The reason for reverting is: this CL doesn't fix the original issue.