Add RemoteFrameView accessor on RemoteFrame.

This was an oversight from r170149.

R=dcheng@chromium.org
BUG=346764

Review URL: https://codereview.chromium.org/265743007

git-svn-id: svn://svn.chromium.org/blink/trunk@173112 bbb929c8-8fbe-4397-9dbb-9b2b20218538