CC Animation: Extract PropertyAnimationState as a non-nested struct.

Next step: Rework UpdateClientAnimationState to deal with
a bitset instead of single property
(batch, batch, batch)

BUG=592873
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2349643003
Cr-Commit-Position: refs/heads/master@{#422704}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 84822d0..2bde91f3 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -28,6 +28,8 @@
     "animation/element_id.h",
     "animation/keyframed_animation_curve.cc",
     "animation/keyframed_animation_curve.h",
+    "animation/property_animation_state.cc",
+    "animation/property_animation_state.h",
     "animation/scroll_offset_animation_curve.cc",
     "animation/scroll_offset_animation_curve.h",
     "animation/scroll_offset_animations.cc",