Web Animations: Refactor KeyframeEffectModel to work via an InterpolationEffect.

This patch modifies KeyframeEffectModel's implementation such that
an InterpolationEffect is generated after Keyframe normalization. The
InterpolationEffect then handles sample generation, and Interpolations are
plumbed down through the style resolution pipeline instead of
CompositableValues.

BUG=346495

Review URL: https://codereview.chromium.org/194673002

git-svn-id: svn://svn.chromium.org/blink/trunk@169500 bbb929c8-8fbe-4397-9dbb-9b2b20218538