Replace high-contrast-mode with dark-mode in Blink's web tests.

This is a follow-up to https://crrev.com/c/1500255.

Bug: 938511
Change-Id: If3a4f8a3488557808551da8180ff858522f23052
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506872
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Auto-Submit: Aran Gilman <gilmanmh@google.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638337}
38 files changed