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After crrev.com/295851 the PipeMessagingChannel verifies that incoming
messages are dictionaries and then passes them as base::Value to
OnMessage() and then OnMessage() implementation static_cast<> them to
base::DictionaryValue. This is fragile, and also it shouldn't be
PipeMessagingChannel's responsibility to verify format of the messages
it reads. Moved the check to OnMessage() implementations.

Review URL: https://codereview.chromium.org/615543004

Cr-Commit-Position: refs/heads/master@{#297274}