Add NULL renderer check when setting layer for a remote frame

Follow-up to r176130. Ojan noted that the Frame might not have an
ownerRenderer, this patch adds a check for that scenario.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/339873003

git-svn-id: svn://svn.chromium.org/blink/trunk@176341 bbb929c8-8fbe-4397-9dbb-9b2b20218538