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The dialog is accessed by clicking on a tab group header. This
preliminary version of the dialog is mainly intended for testing. The
color selection combobox will eventually be replaced with a fancier
color selector.

Change-Id: I7736a0bbeb1af5fb2aaa3b6d3e418008aa72ed11
Bug: 989174
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717552
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683821}