Remove forward declarations of unused ImageRepGdk and ImageRepCairo

ui/gfx/image/image.cc

Index: ui/gfx/image/image.cc
diff --git a/ui/gfx/image/image.cc b/ui/gfx/image/image.cc
index 7bfcacd1bb511e7078511e1062fef574f042bf2d..5c81a5519ed6be6637bfbb8da200dfebd4ba061c 100644
--- a/ui/gfx/image/image.cc
+++ b/ui/gfx/image/image.cc
@@ -102,7 +102,6 @@ scoped_refptr<base::RefCountedMemory> Get1xPNGBytesFromImageSkia(
 
 class ImageRepPNG;
 class ImageRepSkia;
-class ImageRepGdk;
 class ImageRepCocoa;
 class ImageRepCocoaTouch;
 
@@ -697,11 +696,7 @@ void Image::SetSourceColorSpace(CGColorSpaceRef color_space) {
 
 Image::RepresentationType Image::DefaultRepresentationType() const {
   CHECK(storage_.get());
-  RepresentationType default_type = storage_->default_representation_type();
-  // The conversions above assume that the default representation type is never
-  // kImageRepCairo.
-  DCHECK_NE(default_type, kImageRepCairo);
-  return default_type;
+  return storage_->default_representation_type();
 }
 
 internal::ImageRep* Image::GetRepresentation(