Change WebLayer::addAnimation to transfer ownership of WebAnimation.

Introduce temporary macro, ANIMATION_OWNERSHIP_TRANSFER, to change
addAnimation() in both Blink and Chromium. After Chromium is aligned to this CL,
ANIMATION_OWNERSHIP_TRANSFER will be removed.

BUG=290217

Review URL: https://codereview.chromium.org/23834010

git-svn-id: svn://svn.chromium.org/blink/trunk@160110 bbb929c8-8fbe-4397-9dbb-9b2b20218538