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When download is paused, watching the mojo data pipe for data ready
callback is not needed. As the callback will trigger an early return
and do nothing. We can register the callback later when download is
resumed.

BUG=927530

Change-Id: Ifc225241978cb34991846ffb4a6a40166e5fc953
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1748122Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686590}