Web Animations CSS: Fix and clean up compositableValuesFor* methods

The compositableValuesForAnimations and compositableValuesForTransitions
methods were accidentally implemented switched, so that each returns the
CompositableValueMap that the other should have returned. This basically
means that they end up being applied in the wrong order in the cascade.

This patch fixes this behaviour, and adds a test checking animations are
applied before !important. We'll add a similar test for transitions once
the virtual test suite for transitions/web-animations-css is enabled.

We also make some minor clean-ups to simplify the code, passing pointers
to AnimationStack::compositableValues, instead of references and values.
In the normal case, we won't have an update and these will just be null.

BUG=303430, 232273

Review URL: https://codereview.chromium.org/39503002

git-svn-id: svn://svn.chromium.org/blink/trunk@160702 bbb929c8-8fbe-4397-9dbb-9b2b20218538