Support for prefers-color-scheme media feature.

The preferred color scheme can be passed to the renderer through
Settings, but there is no code in content changing it from the default
value yet.

Spec:

https://drafts.csswg.org/mediaqueries-5/#prefers-color-scheme

Intent to Implement:

https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/Muw0N43ntSw

Bug: 889087
Change-Id: I859de2953966640baed57ee6d4a5edcbabf9e999
Reviewed-on: https://chromium-review.googlesource.com/c/1396404
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621528}
19 files changed