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to ensure that the callback function is set before ConnectChannel()
is called. This prevents a potential race condition that could occur
if messages were pending when ipc_adapter->ConnectChannel() is called.

TEST=git cl try, ARC still works, PNaCl demo still works
BUG=nativeclient:3802

Review URL: https://codereview.chromium.org/1090043002

Cr-Commit-Position: refs/heads/master@{#325535}