Allow RemoteFrames to be navigated through targeted navigation.

      Currently, FrameLoader assumes the target frame of a navigation is always
LocalFrame. This is not the case once we have out-of-process iframes.
This patch allows navigation to be sent to RemoteFrame if the frame is in
a different process.

BUG=496923

Review URL: https://codereview.chromium.org/1152053006

git-svn-id: svn://svn.chromium.org/blink/trunk@196689 bbb929c8-8fbe-4397-9dbb-9b2b20218538