Move small c/b/extensions classes into extensions namespace no.2

chrome/browser/ui/extensions/shell_window.h

Index: chrome/browser/ui/extensions/shell_window.h
diff --git a/chrome/browser/ui/extensions/shell_window.h b/chrome/browser/ui/extensions/shell_window.h
index 4d74fc8c720b4409d05e021a9177f8cf3426e005..bc9ee7f94fe83965f982141df5b374d3968dc6a3 100644
--- a/chrome/browser/ui/extensions/shell_window.h
+++ b/chrome/browser/ui/extensions/shell_window.h
@@ -27,6 +27,7 @@ class WebContents;
 namespace extensions {
 class Extension;
 class WindowController;
+class PlatformAppBrowserTest;
 }
 
 // ShellWindow is the type of window used by platform apps. Shell windows
@@ -84,7 +85,7 @@ class ShellWindow : public content::NotificationObserver,
 
  private:
   // PlatformAppBrowserTest needs access to web_contents()
-  friend class PlatformAppBrowserTest;
+  friend class extensions::PlatformAppBrowserTest;
 
   // Instantiates a platform-specific ShellWindow subclass (one implementation
   // per platform). Public users of ShellWindow should use ShellWindow::Create.