Make Keyframe::PropertySpecificKeyframes RefCounted

This change adds ref counting to Keyframe::PropertySpecificKeyframe.
This refactor is in preparation for https://codereview.chromium.org/1698093005
which requires multiple owners of a PropertySpecificKeyframe.
This patch introduces no change in behaviour.

BUG=360473

Review URL: https://codereview.chromium.org/1701813002

Cr-Commit-Position: refs/heads/master@{#375813}
12 files changed