Factor most Dark Mode code out of GraphicsContext.

This is a clean-up CL in preparation for adding an image-specific
filter.

Bug: 954318
Change-Id: I002e09c2ebd384c0f0d667dcb00eebc7550aee71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574629
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652679}
5 files changed