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Created TabGroupViews, which manages most of the tab group's visual calculations and updates. TabStrip still paints the views, but it no longer keeps low-level maps to each individual view.

TabGroupUnderline can now use TabGroupViews for the bulk of its visual calculations. In the future, TabGroupHighlight will take a similar approach.

Change-Id: I894dc03375cb29b42b80bd1273587a41131f1b22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1925336
Commit-Queue: Connie Wan <connily@chromium.org>
Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721202}