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We originally did this to try and ensure tabs had sufficient contrast to be
readable in all cases.  However, this caused problems if the background colors
were not correctly calculated, and also limited theme authors' ability to make
aesthetic choices (or override us if we got something wrong).

Instead, use the specified colors directly.  Generate unspecified inactive tab
colors by blending with the background, as we do for the default theme.  No
changes to how the default theme's colors are calculated.

Bug: 904372
Change-Id: I90d96afbf3a1e9b095b0860a64aa044fecdad6d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1755142
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687315}