Make loader classes aware of RemoteFrames in FrameTree

FrameTree iteration in FrameLoader and other loader classes must
be able to handle the possibility of RemoteFrames appearing in the
tree.

BUG=346764
R=dcheng@chromium.org

Review URL: https://codereview.chromium.org/311663003

git-svn-id: svn://svn.chromium.org/blink/trunk@175377 bbb929c8-8fbe-4397-9dbb-9b2b20218538