Remove obsolete GuestWebContentsObserver reference

BUG=None
Test=None, internal cleanup.

Review URL: https://codereview.chromium.org/411253009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285550 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/guest_view/guest_view_manager.h

@@ -17,7 +17,6 @@
 class AppViewGuest;
 class GuestViewBase;
 class GuestViewManagerFactory;
-class GuestWebContentsObserver;
 class GURL;
 
 namespace content {
@@ -81,7 +80,6 @@ class GuestViewManager : public content::BrowserPluginGuestManager,
  protected:
   friend class AppViewGuest;
   friend class GuestViewBase;
-  friend class GuestWebContentsObserver;
   friend class guestview::TestGuestViewManager;
   FRIEND_TEST_ALL_PREFIXES(GuestViewManagerTest, AddRemove);