CSS Animations: Allow target wrappers in animation test helper templates

Some interpolation tests require the target element be contained in a
parent element e.g. width: 100%.
This patch allows target templates to describe a container for the target
instead of the top level template element being selected as the target
always.

Review URL: https://chromiumcodereview.appspot.com/23510011

git-svn-id: svn://svn.chromium.org/blink/trunk@158082 bbb929c8-8fbe-4397-9dbb-9b2b20218538