Fix WeakPtrFactory members in Android media classes

Fix WeakPtrFactory members in Android media classes

This forces WeakPtrs to be invalidated before other members' destructors run.
See the bug or the documentation of WeakPtrFactory for more information. I'm
in the process of trying to turn on a clang check for this (see the bug).

BUG=303818
TBR=qinmin@chromium.org

Committed: https://crrev.com/2dd23c37a08ad705c9cb48ededb18cce29609aa5
Cr-Commit-Position: refs/heads/master@{#311506}

[dmichael (off chromium), 2015-01-14 16:57:31]

dmichael@chromium.org changed reviewers:
+ teravest@chromium.org
- qinmin@chromium.org

[teravest, 2015-01-14 16:59:12]

lgtm

[dmichael (off chromium), 2015-01-14 17:04:39]

The CQ bit was checked by dmichael@chromium.org

[commit-bot: I haz the power, 2015-01-14 17:05:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/841453006/20001

[commit-bot: I haz the power, 2015-01-14 17:11:48]

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

[commit-bot: I haz the power, 2015-01-14 17:11:49]

Try jobs failed on following builders:
  ios_dbg_simulator on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[dmichael (off chromium), 2015-01-14 17:16:05]

The CQ bit was checked by dmichael@chromium.org

[commit-bot: I haz the power, 2015-01-14 17:16:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/841453006/20001

[commit-bot: I haz the power, 2015-01-14 17:59:59]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-01-14 18:01:41]

Patchset 2 (id:??) landed as
https://crrev.com/2dd23c37a08ad705c9cb48ededb18cce29609aa5
Cr-Commit-Position: refs/heads/master@{#311506}