Web Animations CSS: Disable the matched properties cache when running animations/transitions

This patch disables our usage of the matched properties cache when we
are running animations or transitions. Since we plan to adhere to the
specified cascade order, the matched properties cache will in general
not be applicable (due to the high/low priority dependencies, we'd be
able to cache only the result of applying high priority properties at
levels below animations). We can probably do better in cases where we
only have animations/transitions applying in low priority properties,
where we'd just re-apply those and !important properties, but for now
we'll disable the cache entirely, and we can add this optimisation if
the need arises.

BUG=258896

Review URL: https://codereview.chromium.org/54683004

git-svn-id: svn://svn.chromium.org/blink/trunk@161241 bbb929c8-8fbe-4397-9dbb-9b2b20218538