PPAPI: Don't use MessageLoop::current in the plugin process

PPAPI: Don't use MessageLoop::current in the plugin process

The plugin is allowed to use Pepper without having bound a PPB_MessageLoop
to the calling thread, which means MessageLoop::current won't be valid.
Instead, we should just post to the Pepper main thread.

BUG=459671
Committed: https://crrev.com/ac860c7f9ad3a532a6cf0684aaa270d9c0ef36a6
Cr-Commit-Position: refs/heads/master@{#316924}

[dmichael (off chromium), 2015-02-18 22:00:04]

dmichael@chromium.org changed reviewers:
+ raymes@chromium.org

[raymes, 2015-02-18 22:16:41]

On 2015/02/18 22:00:05, dmichael wrote:

lgtm

[dmichael (off chromium), 2015-02-18 22:36:26]

The CQ bit was checked by dmichael@chromium.org

[commit-bot: I haz the power, 2015-02-18 22:36:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/938843002/1

[commit-bot: I haz the power, 2015-02-18 23:52:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-18 23:53:18]

Patchset 1 (id:??) landed as
https://crrev.com/ac860c7f9ad3a532a6cf0684aaa270d9c0ef36a6
Cr-Commit-Position: refs/heads/master@{#316924}