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These will eventually be used for three purposes:
(1) To allow generating different background tab images from a common tint for
    the inactive and incognito cases, thus allowing background tabs that blend
    into the frame for custom themes.
(2) To allow autogenerating the background tab colors from the background tab
    images, thus letting us autocalculate better tab text/separator/etc. colors.
(3) To allow theme authors to customize/override this behavior more precisely.

Reasons (1) and (2) dictated the design here; reason (3) is basically a free
bonus.  I thought about not making these resources overrideable by theme
authors, but there was some technical reason that turned out to not be easily
feasible, and there seems little reason to prevent it.

This also allows us to read THEME_TAB_BACKGROUND_INCOGNITO on Mac (Views).

This shouldn't result in a visual change on its own.

Bug: 862664
Change-Id: Icc57f78fd526d68b83c692fbac669a7371bb38ff
Reviewed-on: https://chromium-review.googlesource.com/1147869
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578790}