chrome: Cleanup class/struct forward declarations

chrome/browser/media/protected_media_identifier_permission_context.h

Index: chrome/browser/media/protected_media_identifier_permission_context.h
diff --git a/chrome/browser/media/protected_media_identifier_permission_context.h b/chrome/browser/media/protected_media_identifier_permission_context.h
index 6546a7b896e3c9b8b7a36d1dd2d9e175725c2350..8b00f4ab7af5b6bdbd6f461bfcdce62188900042 100644
--- a/chrome/browser/media/protected_media_identifier_permission_context.h
+++ b/chrome/browser/media/protected_media_identifier_permission_context.h
@@ -28,10 +28,6 @@ namespace content {
 class WebContents;
 }
 
-namespace user_prefs {
-class PrefRegistrySyncable;
-}
-
 // Manages protected media identifier permissions flow, and delegates UI
 // handling via PermissionQueueController.
 class ProtectedMediaIdentifierPermissionContext