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Reland https://chromium-review.googlesource.com/804357. This time ensure
that we are using a MouseDown event instead of MouseMove event as
MouseMove events can be sent from the UI synthetically and that causes
the InputMsgWatcher to be in a different state sometimes.

Devtools was not correctly handling input that is not generated by itself
and invoking callbacks when the input wasn't actually processed.

Add a modifier indicating content was injected using the debugger so that
we can then query for that to determine if we should release the event.
This is a stop gap solution until we can actually bind callbacks into the
sending of events and devtools wouldn't need its own queues of callbacks.

BUG=789869
TBR=pfeldman@chromium.org

Change-Id: Id769fa9363debb196d787a73920f24e3a409c68f
Reviewed-on: https://chromium-review.googlesource.com/811833
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522417}