Simplify message parsing.

Previously ProtobufMessageParser was used to parse incoming messages.
Removed it and replaced with ParseMessage() function. This allows to
simplify MessageReader and makes it possible to remove MessageReader
dependency in ChannelDispatcherBase, which will be done later.

Review URL: https://codereview.chromium.org/1654513003

Cr-Commit-Position: refs/heads/master@{#372526}
27 files changed