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Prior to this CL, the tabstrip always occupies the entire width of the
titlebar area (minus the caption buttons). Changing this allows us to
lay out the NTB just to the right of the tabstrip view itself instead
of having the tabstrip control the NTB's bounds. This CL does that
by simply changing the FlexRule governing the tabstrip's width (and
fixing the numerous resulting bugs).

This is a modified reland of https://crrev.com/c/2336296. Changes from
that version:
- PreferredSizeChanged() while dragging, since the dragged tabs may have
been moved past the previous extents of the tabstrip.
- Take tab closing mode into account when determining the tab area's
preferred width.
- Bypass ScrollView when observing the tabstrip's preferred size changes.
- PreferredSizeChanged() in SnapToIdealBounds, since the tabs may have
been snapped outside the previous extents of the tabstrip.

Known issues:
- Tab closing mode doesn't play nicely with scrolling in all cases. That
was true before this CL, but this moves around which cases are broken.

Bug: 1093972
Change-Id: I214534847bd07c53829005b7496a9b9bc63e6d5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2432088
Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816778}