Update FrameTree to use FrameClient via Frame.

FrameTree used to go through FrameLoader to get this information, but
RemoteFrames don't have a FrameLoader, so this patch moves the
FrameClient pointer to the Frame base class and updates FrameLoader to
use it as well.

BUG=346764
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/299703017

git-svn-id: svn://svn.chromium.org/blink/trunk@174715 bbb929c8-8fbe-4397-9dbb-9b2b20218538