Bug 1344839 - Don't allow transparent top-level windows on GTK. r=karlt

MozReview-Commit-ID: 9UP1UYcaF2F

--HG--
extra : rebase_source : 33b5aa13466e642e335a0653f9f436bb64648c42

widget/gtk/nsWindow.cpp

@@ -4303,6 +4303,12 @@ nsWindow::SetTransparencyMode(nsTransparencyMode aMode)
         topWindow->SetTransparencyMode(aMode);
         return;
     }
+
+    if (mWindowType != eWindowType_popup) {
+        NS_WARNING("Cannot set transparency mode on non-popup windows.");
+        return;
+    }
+
     bool isTransparent = aMode == eTransparencyTransparent;
 
     if (mIsTransparent == isTransparent)