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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

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

Cr-Commit-Position: refs/heads/master@{#296311}