Optimize style recalc when adding @keyframes.

We only need to recalculate the elements with running animations unless
we tried to find an @keyframes rule and couldn't, for which we fall
back to a full document recalc.

The motivation for doing this is that the current WIP for issue 567021
shows that we're still getting full document recalcs due to the
presence of @keyframes rules.

R=dstockwell@chromium.org,alancutter@chromium.org
BUG=623911,567021

Review-Url: https://codereview.chromium.org/2105743002
Cr-Commit-Position: refs/heads/master@{#403118}
10 files changed