ui: Cleanup class/struct forward declarations

ui/keyboard/content/keyboard_ui_content.h

Index: ui/keyboard/content/keyboard_ui_content.h
diff --git a/ui/keyboard/content/keyboard_ui_content.h b/ui/keyboard/content/keyboard_ui_content.h
index 29c8a110ba7a5ec9c60e22e65a56372001930614..85a9372d0956985c502a952b3f0db0bd75a88cfd 100644
--- a/ui/keyboard/content/keyboard_ui_content.h
+++ b/ui/keyboard/content/keyboard_ui_content.h
@@ -19,22 +19,17 @@ class Window;
 }
 namespace content {
 class BrowserContext;
-class SiteInstance;
 class WebContents;
 }
 namespace gfx {
 class Rect;
 }
-namespace ui {
-class InputMethod;
-}
 namespace wm {
 class Shadow;
 }
 
 namespace keyboard {
 
-class KeyboardController;
 class WindowBoundsChangeObserver;
 
 // An implementation of KeyboardUI that uses a content::WebContents to implement