Add SourceAlpha support to accelerated CSS filter effects (aka Skia SkImageFilter DAG) path.

BUG=331362

Review URL: https://codereview.chromium.org/123293002

git-svn-id: svn://svn.chromium.org/blink/trunk@164424 bbb929c8-8fbe-4397-9dbb-9b2b20218538