Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

include/views/cef_window_delegate.h

@@ -63,6 +63,21 @@ class CefWindowDelegate : public CefPanelDelegate {
   /*--cef()--*/
   virtual void OnWindowDestroyed(CefRefPtr<CefWindow> window) {}
 
+  ///
+  // Return the parent for |window| or NULL if the |window| does not have a
+  // parent. Windows with parents will not get a taskbar button. Set |is_menu|
+  // to true if |window| will be displayed as a menu, in which case it will not
+  // be clipped to the parent window bounds. Set |can_activate_menu| to false
+  // if |is_menu| is true and |window| should not be activated (given keyboard
+  // focus) when displayed.
+  ///
+  /*--cef()--*/
+  virtual CefRefPtr<CefWindow> GetParentWindow(CefRefPtr<CefWindow> window,
+                                               bool* is_menu,
+                                               bool* can_activate_menu) {
+    return NULL;
+  }
+
   ///
   // Return true if |window| should be created without a frame or title bar. The
   // window will be resizable if CanResize() returns true. Use