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There are actually two issues fixed by this CL. The first is the tab
strip reappearing after backgrounding/switching dark mode. This happens
because the BVC currently just sets the hidden status when revealing the
thumb strip and when the trait collection changes, so unhiding on trait
collection change overrides the hiding from revealing the thumb strip.
This is fixed by tracking these two separately in the tab strip
controller and using both to determine the overall hidden state.

Second is the thumb strip breaking on backgrounding. This happens
because I slightly misunderstood the UIGestureRecognizerDelegate.
-gestureRecognizerShouldBegin: doesn't necessarily mean WILL begin, so
the |currentRecognizer| should only be set once the pan gesture
actually begins.

Fixed: 1166020, 1165984
Change-Id: I177231f8be13fbc372358b2648a5a0d0a15d7f7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626390Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#843110}