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Each POSIX(ish) platform uses a different underlying MessagePumpForIO
implementation, but all are required to implement a common interface
to support asynchronous I/O on file descriptors.

This CL adds a set of MessageLoopIoPosixTests, which exercise those
functions to verify the semantics of all the implementations. These
include existing tests moved from MessageLoopTest and
MessagePumpLibeventTest, and a new test for mid-callback teardown.

Bug: 
Change-Id: I2b7071107342489530076dfc611738a7ec39065c
Reviewed-on: https://chromium-review.googlesource.com/571518
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487337}