Have Element::ensureUserAgentShadowRoot() return a reference

Have Element::ensureUserAgentShadowRoot() return a reference instead of
a pointer as it can never return null.

R=adamk

Review URL: https://codereview.chromium.org/59463007

git-svn-id: svn://svn.chromium.org/blink/trunk@161631 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove InbandTextTrackPrivate/InbandTextTrackPrivateClient

These interfaces were virtually identical to the Web* variants, so
just use those directly.

BUG=316172

Review URL: https://codereview.chromium.org/60083011

git-svn-id: svn://svn.chromium.org/blink/trunk@161632 bbb929c8-8fbe-4397-9dbb-9b2b20218538