Web Animations CSS: Support animation of stroke-opacity

Note that the existing implementation fails to clamp animated values to the
correct range. Behavior under web-animations-css matches Gecko.

BUG=257591

Review URL: https://codereview.chromium.org/25184003

git-svn-id: svn://svn.chromium.org/blink/trunk@158599 bbb929c8-8fbe-4397-9dbb-9b2b20218538