Web Animations CSS: Implement AnimatableValue::equals

This function is intended to be used for the upcoming transitions
implementation on the Web Animations model. This will be used for
seeing if the computed value for an element has been updated, and
comparing to the start and goal of running transitions.

This is slightly more expensive than comparing values directly off
RenderStyles (CSSPropertyAnimation), but is probably a cleaner way
of implementing this (we can later see if the extra indirection is
going to affect performance, in particular for transition all).

The behaviour here on comparing ShapeValue and ClipPathOperation is
slightly changed from CSSPropertyAnimation, and we now will compare
the objects themselves instead of just pointers.

BUG=303430

Review URL: https://codereview.chromium.org/25813004

git-svn-id: svn://svn.chromium.org/blink/trunk@159097 bbb929c8-8fbe-4397-9dbb-9b2b20218538