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Prior to this CL, there was a magic 166 alpha applied to the
system focus ring color (the Accent Color setting). This has
now been pulled into native_theme.

Along the way, I also added a Color::operator SkColor() to
convert blink::Color to SkColor, and used it in gradient.cc,
along with here in layout_theme_mac.mm. I'm sure there are
other such converters sprinkled around which could be
migrated, but this was all I found.

Quick note on testing: this change will be best tested by [1]
once it lands.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2150765

Fixed: 1108169
Change-Id: I54ac4e60a1df1b39123721adffda9f39aac94fe4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317294
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792223}