AW: add debug flag to tint all WebViews

No change to default code path. This adds a debugging flag to tint all
WebViews yellow. This exposes the flag through the developer UI and
documents the flag.

Bug: 1051554
Test: build/android/adb_system_webview_command_line --highlight-all-webviews
Test: out/Default/bin/system_webview_shell_apk launch "https://developer.mozilla.org/en-US/"
Change-Id: Ib1cf2191cd51cb793d040ce9e8d5eb9e7ecebe68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066144


Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743329}