Web Animations CSS: Cancel animations on children of elements that become display:none.

r155386[1] assumed that context.performingReattach would be true iff a renderer
would be subsequently created after detach(). performingReattach does not
signal this.

This patch adds a check to see whether createRendererForElementIfNeeded()
creates a renderer in attach(), and cancels animations if no renderer exists.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=155386

BUG=327492

Review URL: https://codereview.chromium.org/113293003

git-svn-id: svn://svn.chromium.org/blink/trunk@163742 bbb929c8-8fbe-4397-9dbb-9b2b20218538