[Mac] Make BookmarkBubbleController is-a BaseBubbleController.

chrome/browser/ui/cocoa/extensions/extension_installed_bubble_controller.mm

Index: chrome/browser/ui/cocoa/extensions/extension_installed_bubble_controller.mm
diff --git a/chrome/browser/ui/cocoa/extensions/extension_installed_bubble_controller.mm b/chrome/browser/ui/cocoa/extensions/extension_installed_bubble_controller.mm
index 231fa62f354460f109ae3e0207eb38cc18af1d26..9d4317ec70e91d2c67fc99beab6292da06c34523 100644
--- a/chrome/browser/ui/cocoa/extensions/extension_installed_bubble_controller.mm
+++ b/chrome/browser/ui/cocoa/extensions/extension_installed_bubble_controller.mm
@@ -221,12 +221,8 @@ class ExtensionLoadedNotificationObserver
   return arrowPoint;
 }
 
-// We want this to be a child of a browser window.  addChildWindow:
-// (called from this function) will bring the window on-screen;
-// unfortunately, [NSWindowController showWindow:] will also bring it
-// on-screen (but will cause unexpected changes to the window's
-// position).  We cannot have an addChildWindow: and a subsequent
-// showWindow:. Thus, we have our own version.
+// Override -[BaseBubbleController showWindow:] to tweak bubble location and
+// set up UI elements.
 - (void)showWindow:(id)sender {
   // Generic extensions get an infobar rather than a bubble.
   DCHECK(type_ != extension_installed_bubble::kGeneric);