GitLab

Adds Attach/Detach as virtual methods on MessagePumpCFRunLoopBase
so that they can be called unconditionally from SequenceManagerImpl.

BUG=1002087

Change-Id: I8263188aa388a449c799b7bde17e7dac8b44d3be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1826517
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718316}