Bug 1522780 - [Wayland] Use MOZ_ENABLE_WAYLAND to enable wayland backend, r=glandium

Recently we use GDK_BACKEND to enable/disable Wayland backend. That's good for testing but bad for distro deployment.

When GDK_BACKEND is set it's propagated to child processes which may not support wayland thus they fail to run. Also when GDK_BACKEND=wayland is set Firefox fails to start when Wayland backend is not available.

To allow easy deployment let's use a specific MOZ_ENABLE_WAYLAND env which means to use a default available GTK backend (x11 or wayland) and don't fail when Wayland is missing.

Differential Revision: https://phabricator.services.mozilla.com/D17607

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

toolkit/xre/nsAppRunner.cpp

@@ -3622,12 +3622,15 @@ int XREMain::XRE_mainStartup(bool* aExitFlag) {
 
     bool disableWayland = true;
 #  if defined(MOZ_WAYLAND)
-    // Make X11 backend the default one.
-    // Enable Wayland backend only when GDK_BACKEND is set and
-    // Gtk+ >= 3.22 where we can expect recent enough
+    // Enable Wayland on Gtk+ >= 3.22 where we can expect recent enough
     // compositor & libwayland interface.
-    disableWayland = (PR_GetEnv("GDK_BACKEND") == nullptr) ||
-                     (gtk_check_version(3, 22, 0) != nullptr);
+    disableWayland = (gtk_check_version(3, 22, 0) != nullptr);
+    if (!disableWayland) {
+      // Make X11 backend the default one unless MOZ_ENABLE_WAYLAND or
+      // GDK_BACKEND are specified.
+      disableWayland = (PR_GetEnv("GDK_BACKEND") == nullptr) &&
+                       (PR_GetEnv("MOZ_ENABLE_WAYLAND") == nullptr);
+    }
 #  endif
     // On Wayland disabled builds read X11 DISPLAY env exclusively
     // and don't care about different displays.