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Currently cc::AnimationTarget is implemented by cc::ElementAnimation,
vr::UiElement and vr::TestAnimationTarget. Make it abstract for better
readability.

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:linux_vr;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ic46040c4f703de76058beec68789da810a8ac53c
Reviewed-on: https://chromium-review.googlesource.com/1131228Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580366}