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The issue (crbug.com/719011) indicate that it is hard to interact with
the content under the the scrollbar because the scrollbar will fade in
while mouse moves in the hover fade in region of scrollbar.

In this patch, we prevent fade in overlay scrollbar when user mouse
down. 3 behaviors changed:

1. When mouse move in hover fade in region of scrollbar with mouse
   press, we don't trigger delay fade in.
2. When mouse down after a delay fade in triggered, cancel the delay
   fade in.
3. When mouse up in hover fade in region of scrollbar, trigger a delay
   fade in.

BUG=719011
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2931703002
Cr-Commit-Position: refs/heads/master@{#478733}