Patch overlay panel event stream internally

This change adds logic inside the OverlayPanelEventFilter to make
sure that at least one down event is being passed to the internal
gesture detector before a stream starts.

BUG=715183

Review-Url: https://codereview.chromium.org/2843263003
Cr-Commit-Position: refs/heads/master@{#467768}
1 file changed