Support colors w/ alpha in SVG 'stop' elements and 'feFlood'

When a color w/ alpha is specified (via rgba(...)) as stop-color on
a SVG 'stop' element, or as flood-color (or equivalent) on 'feFlood',
then modulate the alpha-channel with the stop-opacity or flood-opacity
respectively to produce the final color to use.

BUG=331516

Review URL: https://codereview.chromium.org/126033003

git-svn-id: svn://svn.chromium.org/blink/trunk@164690 bbb929c8-8fbe-4397-9dbb-9b2b20218538