Add Dark Mode web tests for inverting pages based on background color.

Bug: 925949
Change-Id: Ie6042612b107874f984a0ae03794ec235b3e6cc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1510114
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642044}
14 files changed