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This adds postMessageAndAwaitResponse() support to Pepper stable.

Open issues:
1) This is arguably an expansion on what JavaScript can do today. In particular,
it would be possible to make JavaScript block on PNaCl, which blocks on another
thread, which blocks on something like synchronous FileIO.
2) Not all types are supported yet; e.g. FileRef and FileSystem. I want to fix
that prior to M39. (see https://code.google.com/p/chromium/codesearch#chromium/src/content/renderer/pepper/message_channel.cc&l=305)

BUG=367896

Review URL: https://codereview.chromium.org/600893002

Cr-Commit-Position: refs/heads/master@{#297026}