Web Animations API: Length properties responsive to style changes

Where start and end length properties are specified as percentages, or using units that depend on other properties, we now avoid capturing state when the animation is created.

BUG=361948

Review URL: https://codereview.chromium.org/273903002

git-svn-id: svn://svn.chromium.org/blink/trunk@174647 bbb929c8-8fbe-4397-9dbb-9b2b20218538