Refactor RemoteFrame event handling to remove Document dependency.

RemoteFrames should not have Documents, so it is problematic to have
input event forwarding in Document::defaultEventHandler.

This change moves the handling for most events to HTMLFrameElementBase,
except for keyboard events which are forwarded before reaching
EventHandler if a remote frame has focus.

Frame::isRemoteFrameTemporary() is also added to identify LocalFrames
that are being used to represent remote frames, which is necessary
until RemoteFrame can be instantiated.

BUG=346764
R=dcheng@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/227703007

git-svn-id: svn://svn.chromium.org/blink/trunk@171078 bbb929c8-8fbe-4397-9dbb-9b2b20218538