cc: Allow transparent layers with background filters to render

If we have a background filter, we still need to render the fully
transparent stuff since the filter will still change what's beneath it.

+test

BUG=497522
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1307713002

Cr-Commit-Position: refs/heads/master@{#344912}
3 files changed