Just linearly search for keyframes in the set of stylesheets.
Components rarely have many sheets, and sheets rarely have many
keyframes so this should be quite fast. It's also much simpler
than having to collect all the keyframes from all the rulesets.
R=ojan@chromium.org, rafaelw@chromium.org
Review URL: https://codereview.chromium.org/839473005
8b97399df3