Move AnimationValue onto the heap

Move AnimationValue onto the heap

This changes AnimationValue from a struct on the stack to being an
object on the heap. This simplifies the implementation of AnimationValue
as it no longer needs to reimplement nullptr behaviours and makes
future use of this class simpler by reusing the move operations
provided by OwnPtr.

This change makes FlipPrimitiveInterpolation::Side redundant and removes it.

BUG=437696
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197963

[alancutter (OOO until 2018), 2015-06-26 04:29:09]

alancutter@chromium.org changed reviewers:
+ dstockwell@chromium.org, shans@chromium.org

[alancutter (OOO until 2018), 2015-06-26 04:29:49]

This patch is split out from https://codereview.chromium.org/1215563002.

[shans, 2015-06-29 01:30:28]

lgtm++, nice cleanup

[alancutter (OOO until 2018), 2015-06-29 04:03:52]

The CQ bit was checked by alancutter@chromium.org

[commit-bot: I haz the power, 2015-06-29 04:04:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1214593003/1

[commit-bot: I haz the power, 2015-06-29 05:43:26]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197963