Bug 1417933 - enable CSD titlebar rendering mode on Unity desktop, r=jhorak

MozReview-Commit-ID: AWKJWnsrb7i

--HG--
extra : rebase_source : 5408017b0852cdabc42103c9352d6333df60799b

widget/gtk/nsWindow.cpp

@@ -6876,6 +6876,8 @@ nsWindow::GetCSDSupportLevel() {
             sCSDSupportLevel = CSD_SUPPORT_NONE;
         } else if (strstr(currentDesktop, "MATE") != nullptr) {
             sCSDSupportLevel = CSD_SUPPORT_FLAT;
+        } else if (strstr(currentDesktop, "Unity") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_FLAT;
         } else {
             sCSDSupportLevel = CSD_SUPPORT_NONE;
         }