Make WebContents' theme color a base::Optional<SkColor>

This CL removes the SK_ColorTRANSPARENT sentinel value for
WebContents::GetThemeColor() and updates the return type to be
base::Optional<SkColor>.

This more accurately represents the possible states the
WebContents may be in with regard to the site's theme_color meta
tag.

There are no behavioural changes in this CL.

Bug: 872121
Change-Id: I49a7cc793c0ece326a3bf85c19053d03af5e2c30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1540022
Commit-Queue: Alan Cutter <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Reviewed-by: Jochen Eisinger <[email protected]>
Auto-Submit: Alan Cutter <[email protected]>
Cr-Commit-Position: refs/heads/master@{#645643}
18 files changed