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This addresses #7 and #8 in the attached bug.

Here we selectively repaint only the necessary underline because otherwise there can be weird interactions where the dragged tabs appear underneath unassociated group underlines.

Bug: 1008438
Change-Id: I63038a958249800a2d3df97d8609b8dfe5963410
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846045Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Connie Wan <connily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704219}