[iOS] Update the MainContentUIState upon initialization.

The MainContentUIState used to broacast information to FullscreenModel
is instantiated when the BVC's view appears.  When presenting a view
controller, it receives the hidden/appearing signals and the default
state gets broadcasted.  This CL updates the
WebScrollViewMainContentUIForwarder to update the state immediately if
there is an active WebState.

Bug: 811729
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I823f01d3c86c0ec8dd0dc2b62b873071a0578b16
Reviewed-on: https://chromium-review.googlesource.com/917124
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#536515}(cherry picked from commit a88559f1ee9968255dfcdbd1d02303e2a42729eb)
Reviewed-on: https://chromium-review.googlesource.com/932483
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/branch-heads/3325@{#553}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
1 file changed