Update system colors dynamically

This change exposes LayoutTheme::PlatformColorsDidChange() through
WebRenderTheme. This allows system colors to update dynamically when
new colors are sent from the browser process.

Bug: 970285
Change-Id: Ic3b75dfec668dc5523b0e12765574e66dd35ff8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1797057


Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#700308}