WebUI: Increase contrast for focused cr-action-menu option.

Fixed: 1107917
Change-Id: Ia07198fc4301bf8903586ea16a9f25f5257a1a90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2392978


Commit-Queue: dpapad <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804417}