HistoryController shouldn't call confusingAndOftenMisusedAttached

It's not entirely clear to me why this check is needed at all.  I tried just
removing it entirely but that caused a test to fail. Given that we just got the
document from a frame, we must be worried about the case where we're in the
middle of unloading the document.

Regardless, checking whether the document has a RenderView is more direct than
asking whether it is "attached".

R=japhet,esprehn
BUG=299011

Review URL: https://chromiumcodereview.appspot.com/24976002

git-svn-id: svn://svn.chromium.org/blink/trunk@158515 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Update CSSAnimationUpdate and CSSAnimations to handle multiple animations with a given name

This will be required to handle CSS animations with incomplete keyframes, which we will handle by splitting the animation up into multiple InertAnimation objects.

Uploading this preparatory work as a separate change to simplify the change for incomplete keyframes.

No functional change.

BUG=240653
R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/25079006

git-svn-id: svn://svn.chromium.org/blink/trunk@158516 bbb929c8-8fbe-4397-9dbb-9b2b20218538