Merge to 39: PPAPI: Never re-enter JavaScript for PostMessage.

Merge to 39: 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
TBR=raymes

Committed: https://crrev.com/f73075c99b5ba30e8d62dc5f13fdfb210d0fc506
Cr-Commit-Position: refs/heads/master@{#296311}

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

Review URL: https://codereview.chromium.org/589213003

Cr-Commit-Position: refs/heads/master@{#297308}
(cherry picked from commit 6b328f3db4b55fa299b6cfac9a30efbc47a2ad26)

Committed: https://chromium.googlesource.com/chromium/src/+/541bc0b0a7ab7e6bf81927a3bef0e3068b34099a

[dmichael (off chromium), 2014-10-01 17:39:40]

Committed patchset #1 (id:1) manually as
541bc0b0a7ab7e6bf81927a3bef0e3068b34099a (presubmit successful).