Bug 1600414 [Linux] Apply window aspect ratio to PIP on Gnome only, r=jhorak

Window aspect ratio is known to work correctly on Gnome only so apply it only there.

Differential Revision: https://phabricator.services.mozilla.com/D62548

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

widget/gtk/nsWindow.cpp

@@ -7769,6 +7769,13 @@ GtkTextDirection nsWindow::GetTextDirection() {
 }
 
 void nsWindow::LockAspectRatio(bool aShouldLock) {
+  static const char* currentDesktop = getenv("XDG_CURRENT_DESKTOP");
+  static bool setLock =
+      currentDesktop ? (strstr(currentDesktop, "GNOME") != nullptr) : false;
+  if (!setLock) {
+    return;
+  }
+
   if (aShouldLock) {
     float width = (float)DevicePixelsToGdkCoordRoundDown(mBounds.width);
     float height = (float)DevicePixelsToGdkCoordRoundDown(mBounds.height);