Web Animations - responsive interpolation

Whenever inheriting to and from CSS values like inherit or currentColor, or compound values containing font-dependent lengths, we avoid capturing the styles up-front.

Instead, we only obtain the start and end styles when it is time to interpolate between them.

BUG=361948

Review URL: https://codereview.chromium.org/292173009

git-svn-id: svn://svn.chromium.org/blink/trunk@174900 bbb929c8-8fbe-4397-9dbb-9b2b20218538