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When the selected tab in a tab group changes, the length of the group
underline may need to change. This was not happening, resulting in the
attached bug.

This CL refactors TabStrip::SetSelection() in a handful of small ways:
- makes old selected tab and new selected tab explicit local variables
- simplifies conditional logic slightly
- makes an implicit assumption that new selected tab is valid explicit
  * there had been a check in one place for an invalid index
  * however, it just blindly dereferences it later in the function which
    would have caused a crash
- when active tab changes, if either tab was in a group, recalculates
  group underline and highlight bounds via UpdateTabGroupVisuals()

Fixed: 1098392
Change-Id: Ib4b41bd8ce3e7c601d0329cef9d96b8d5846d7b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261166
Commit-Queue: Dana Fried <dfried@chromium.org>
Commit-Queue: Connie Wan <connily@chromium.org>
Auto-Submit: Dana Fried <dfried@chromium.org>
Reviewed-by: Connie Wan <connily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781533}