Check darkModePagePolicy flag before inverting site colors.

Bug: 925949
Change-Id: I55a9d8280be7cbe435f7c84629b79e919fdff016
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1499955
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638311}
2 files changed