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This CL updates the behavior of fullscreen such that the toolbars will
be re-shown when the user starts a scroll past the bottom of a page
when the toolbars are completely collapsed.

Bug: 841094
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I9e2cb3a59e931f83bb43e8a68df48a80bb6d0c3b
Reviewed-on: https://chromium-review.googlesource.com/1200563
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589357}