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The NTB is instead laid out just right of the tabstrip by
TabStripRegionView. Behavior is unchanged, except for:
- The NTB will never hide (e.g. during drag sessions) because
it's now positioned reasonably during the cases where it was
previously hidden.
- The NTB is positioned slightly further to the right. It was
previously overlapping the rightmost tab a bit to be better
aligned relative to the trailing separator. That of course
doesn't make sense when the tabstrip is scrollable, but still
could before that state is reached, or if the tabstrip is
scrolled all the way to the end.
- A few details of the NTB's movement during animations are
different, e.g. when pinning the rightmost tab.

Under the hood, the NTB is no longer explicitly animated;
that is happening implicitly as the tabstrip's bounds change
during animation.

Also rewrites BrowserNonClientFrameViewTabbedTest.HitTestTabstrip
because it was a) testing very different things per platform and
b) was very unclear about what it was expecting and why.

Bug: 1093972
Change-Id: I3cf086a8f9b4219c5ce46b13ed28c9f8aa52e8ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490554
Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824182}