Do not forward declare OBJC types.

We have to disable forward declaration of objective-c
types in Qt when building chromium code.
Chromium does have different, incompatible
forward declarations.

Change-Id: Ib7f7e9839e5c85d758d1e5bb921e67c57a55c4ef
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>