Switch callers of shadow()->oldestShadowRoot() to userAgentShadowRoot()

There's no reason to use this pattern now that userAgentShadowRoot() exists, and
using it looks sketchy since if there was a non-user agent shadow root as the oldest
we'd not ASSERT or crash and instead the code would just allow an author to make us do
something unexpected.

Review URL: https://chromiumcodereview.appspot.com/22334002

git-svn-id: svn://svn.chromium.org/blink/trunk@155654 bbb929c8-8fbe-4397-9dbb-9b2b20218538