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_menu_button_delegate.h

@@ -42,6 +42,12 @@
 
 class CefMenuButton;
 
+///
+// MenuButton pressed lock is released when this object is destroyed.
+///
+/*--cef(source=library)--*/
+class CefMenuButtonPressedLock : public CefBaseRefCounted {};
+
 ///
 // Implement this interface to handle MenuButton events. The methods of this
 // class will be called on the browser process UI thread unless otherwise
@@ -51,12 +57,16 @@ class CefMenuButton;
 class CefMenuButtonDelegate : public CefButtonDelegate {
  public:
   ///
-  // Called when |button| is pressed. Call CefMenuButton::ShowMenu() to show the
-  // resulting menu at |screen_point|.
+  // Called when |button| is pressed. Call CefMenuButton::ShowMenu() to show a
+  // popup menu at |screen_point|. When showing a custom popup such as a window
+  // keep a reference to |button_pressed_lock| until the popup is hidden to
+  // maintain the pressed button state.
   ///
   /*--cef()--*/
-  virtual void OnMenuButtonPressed(CefRefPtr<CefMenuButton> menu_button,
-                                   const CefPoint& screen_point) = 0;
+  virtual void OnMenuButtonPressed(
+      CefRefPtr<CefMenuButton> menu_button,
+      const CefPoint& screen_point,
+      CefRefPtr<CefMenuButtonPressedLock> button_pressed_lock) = 0;
 };
 
 #endif  // CEF_INCLUDE_VIEWS_CEF_MENU_BUTTON_DELEGATE_H_