[CompositeClipPathAnimations] Fix a crash when a keyframe in an animation has clip-path: none

[chromium-wpt-export-bot]

When composited clip path animations are enabled, clip path animations that have a keyframe with clip-path: none crash.

This change adds a filter function to GetAnimationIfCompositable to exclude animations with clip-path: none.

A clip path of none is difficult to paint with composited clip path as currently implemented, and there is no great need to composite this animation given there is no continuous interpolation to be done.

Bug: 1345977 Change-Id: I2bed8ab0ed6a41b7341f92a3bd48cc2e6457fcb4

Reviewed-on: https://chromium-review.googlesource.com/3777767 WPT-Export-Revision: 41afe1bcbcd51389153af1983f0e12c6bf141471