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Allow mousedown to prevent default the back/forward actions. This feature
is controlled via the ExtendedMouseButtons feature flag which is by
default disabled. Since the button events aren't sent from the browser
when the feature is off the blink code doesn't require a feature flag
since it won't be executed.

Intent to Implement: https://groups.google.com/a/chromium.org/d/msg/blink-dev/3DjSQKQzKz8/FI9qbxHhAwAJ

BUG=680741

Change-Id: If9a59fbb085104b26d25e050f1ca7349756b102b
Reviewed-on: https://chromium-review.googlesource.com/781999
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521475}