Have Frame::tree() return a reference

Have Frame::tree() return a reference as it can never return null.
This makes the calling code look safer.

BUG=

Review URL: https://codereview.chromium.org/33353003

git-svn-id: svn://svn.chromium.org/blink/trunk@160216 bbb929c8-8fbe-4397-9dbb-9b2b20218538