Revert of PPAPI: Never re-enter JavaScript for PostMessage.

Revert of PPAPI: Never re-enter JavaScript for PostMessage. (patchset #4 id:60001 of https://codereview.chromium.org/589213003/)

Reason for revert:
May be crashing Chrome:
https://code.google.com/p/chromium/issues/detail?id=417511

Original issue's description:
> PPAPI: Never re-enter JavaScript for PostMessage.
>
> Blocking renderer->plugin messages can be interrupted by any message
> from the plugin->renderer (even async ones). So while handline a blocking
> message, such as HandleInputEvent or HandleBlockingMessage, it's currently
> possible to re-enter JavaScript. This patch makes that impossible by
> queueing up Plugin->Renderer messages sent via PPB_Messaging::PostMessage
> while any renderer->plugin sync message is on the stack.
>
> BUG=384528
>
> Committed: https://crrev.com/f73075c99b5ba30e8d62dc5f13fdfb210d0fc506
> Cr-Commit-Position: refs/heads/master@{#296311}

TBR=raymes@chromium.org,dmichael@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=384528
Committed: https://crrev.com/50c6f1a0cf5d6a991f7005ba8c0e2d2a4a2f6f22
Cr-Commit-Position: refs/heads/master@{#296571}

[bbudge, 2014-09-24 23:41:41]

Created Revert of PPAPI: Never re-enter JavaScript for PostMessage.

[commit-bot: I haz the power, 2014-09-24 23:42:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/601923003/1

[commit-bot: I haz the power, 2014-09-24 23:43:44]

Committed patchset #1 (id:1) as b54d460a861730ad5c422f9533f82338a730738d

[commit-bot: I haz the power, 2014-09-24 23:47:19]

Patchset 1 (id:??) landed as
https://crrev.com/50c6f1a0cf5d6a991f7005ba8c0e2d2a4a2f6f22
Cr-Commit-Position: refs/heads/master@{#296571}