Bug 1420820 - Added MOZ_GTK_TITLEBAR_DECORATION to override window manager detection and set window decoration type, r=jhorak

Enables override window manager default with those values:

MOZ_GTK_TITLEBAR_DECORATION=none - Firefox does not mess with decoration
MOZ_GTK_TITLEBAR_DECORATION=client - Firefox tries to disable titlebar rendering and draws shadows by client side decorations.
MOZ_GTK_TITLEBAR_DECORATION=system - Firefox tries to disable titlebar rendering and leave system (window manager) to draw window decorations.

MozReview-Commit-ID: G60QS3g1TD0

--HG--
extra : rebase_source : fd7b5d2b0282fbd54046947d210b1288a0610a23

widget/gtk/nsWindow.cpp

@@ -6940,6 +6940,19 @@ nsWindow::GetCSDSupportLevel() {
     if (sCSDSupportLevel != CSD_SUPPORT_UNKNOWN) {
         return sCSDSupportLevel;
     }
+
+    const char* decorationOverride = getenv("MOZ_GTK_TITLEBAR_DECORATION");
+    if (decorationOverride) {
+        if (strcmp(decorationOverride, "none") == 0) {
+            sCSDSupportLevel = CSD_SUPPORT_NONE;
+        } else if (strcmp(decorationOverride, "client") == 0) {
+            sCSDSupportLevel = CSD_SUPPORT_FLAT;
+        } else if (strcmp(decorationOverride, "system") == 0) {
+            sCSDSupportLevel = CSD_SUPPORT_FULL;
+        }
+        return sCSDSupportLevel;
+    }
+
     const char* currentDesktop = getenv("XDG_CURRENT_DESKTOP");
     if (currentDesktop) {
         if (strstr(currentDesktop, "GNOME") != nullptr) {