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This is to support mojo sockets. With mojo sockets, we always try to read from
mojo data pipes, and we need a way to cancel pending reads without having
buffered data in socket subclasses.

Bug: 875855
Change-Id: I1b34c15ff9b848517185caa222cce9109497b78e
Reviewed-on: https://chromium-review.googlesource.com/1183996
Commit-Queue: Helen Li <xunjieli@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585184}