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

Revert of PPAPI: Never re-enter JavaScript for PostMessage. (patchset #5 id:80001 of https://codereview.chromium.org/589213003/)

Reason for revert:
Still causing a crash in Canary

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}
>
> Committed: https://crrev.com/3fe4ceee750b2cd130bd402de3d371d8518c3eba
> Cr-Commit-Position: refs/heads/master@{#296807}

TBR=raymes@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=384528
Committed: https://crrev.com/1ea0de3d15fbfc0018ebf473f4d77787d6420c41
Cr-Commit-Position: refs/heads/master@{#297030}

[dmichael (off chromium), 2014-09-26 21:08:18]

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

[commit-bot: I haz the power, 2014-09-26 21:09:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/605393002/1

[commit-bot: I haz the power, 2014-09-26 21:10:37]

Committed patchset #1 (id:1) as 22dc5bdef2cf3d75256b3d2baf94265179acef74

[commit-bot: I haz the power, 2014-09-26 21:11:02]

Patchset 1 (id:??) landed as
https://crrev.com/1ea0de3d15fbfc0018ebf473f4d77787d6420c41
Cr-Commit-Position: refs/heads/master@{#297030}