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* Use tints to compute frame colors in more cases
* Pull dark mode computation out to a temp to make it easy to reuse later
* Separate "shouldn't theme" and "should theme but can't" frame cases

Bug: none
Change-Id: I093e0e88c99fc061a04be69e15203ee3f16d2fb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1941169
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719945}