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The test has been broken since MojoBootstrap was introduced.
The change makes it impossible to inject failure from the ChannelMojo
subclass.

This change uses large data as pending message to trigger failure,
instead of overloading IPC::ChannelMojo functions.

R=viettrungluu@chromium.org
BUG=417439

Review URL: https://codereview.chromium.org/598093006

Cr-Commit-Position: refs/heads/master@{#296565}