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This CL contains several fixes related to prerendered pages:
- Updates the scroll view padding for replaced Tabs.
- Updates FullscreenWebStateListObserver, WebScrollViewMainContentUIForwarder,
  and LegacyToolbarUIUpdater to account for replaced active WebStates.
- Updates FullscreenWebStateObserver to stop observing destroyed WebStates.
- Updates FullscreenWebStateObserver to use ScopedFullscreenDisablers to
  prevent unbalanced increment/decrement disabled counter calls.
- Updates DCHECKs in MainContentUIState that failed for side-swipes.
- Creates TestFullscreenController for internal disabling tests.


Bug: 793891, 793966, 793989
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I7ed1cd0607e23f34f0991b06386d7d66bf8d86ed
Reviewed-on: https://chromium-review.googlesource.com/826266
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524564}