GitLab

Add content layer and corresponding getters in AshColorProvider.
See info here
https://docs.google.com/spreadsheets/d/1ozIoqVHPafIe3HLVLCxY68S8OCnMCP6fw7qWRnMS2ss/edit?ts=5d0ac331#gid=0

See the colors for text and icon here:
https://spec.googleplex.com/crosux-spec/style/typography.html#typography-typescale-surfaces-colors

And use them to get the icon and text colors for system tray.
For icons, since their colors are the same as in AshColorProvider,
use GetContentLayerColor(*) directly.
For texts, their colors are different with colors of
ContentLayerType::kText*, use DeprecatedGetContentLayerColor to keep
the default colors.

Bug: 972162, 982950
Change-Id: I5d0565aba253a341de1bc060417082b000448801
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752114
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687440}