Resolvesbrave/brave-browser#54736
Chromium renders a solid rounded-rect background when a page action chip expands
to show its label. Brave doesn't use this background, so override
GetBackgroundColor() in the chromium_src layer to return SK_ColorTRANSPARENT
instead of falling through to the Chromium color token.
Per-action overrides via PageActionController::OverrideChipColors() are still
respected — the override path is checked first and takes precedence over the
transparent default.
Also, don't use tonal color for PageActionView's bg/fg colors when expanded.
Simon Hong
1c59e73d37