GitLab

There are a few MessagePipeReader sublcasses defined as inter-classes
of ChannelMojo. This makes it harder to hook them for unit testing.
This CL moves these classes their own file. Now unittest can
define subclasses of these to hook some behavor for testing.

This is a preparation for a crash fix.

BUG=410813
TEST=none
R=viettrungluu@chromium.org,yzshen@chromium.org

Review URL: https://codereview.chromium.org/559723002

Cr-Commit-Position: refs/heads/master@{#294439}