Bug 1350875 part 3 - Have GTK notify fullscreen will change. r=karlt

Differential Revision: https://phabricator.services.mozilla.com/D68680

--HG--
extra : moz-landing-system : lando

widget/gtk/nsWindow.cpp

@@ -5715,6 +5715,11 @@ nsresult nsWindow::MakeFullScreen(bool aFullScreen, nsIScreen* aTargetScreen) {
     return NS_ERROR_NOT_AVAILABLE;
   }
 
+  bool wasFullscreen = mSizeState == nsSizeMode_Fullscreen;
+  if (aFullScreen != wasFullscreen && mWidgetListener) {
+    mWidgetListener->FullscreenWillChange(aFullScreen);
+  }
+
   if (aFullScreen) {
     if (mSizeMode != nsSizeMode_Fullscreen) mLastSizeMode = mSizeMode;