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This patch updates the animation interpolation and composition test helper
to provide a neutralKeyframe sentinal value for animating to/from a neutral
keyframe.
Previously an empty string was used which acted as a neutral keyframe by
virtue of failing to parse as a value. This refactor makes it more explicit
when neutral keyframes are being tested.

BUG=437696

Review URL: https://codereview.chromium.org/1881333002

Cr-Commit-Position: refs/heads/master@{#386922}