Bug 1644271 - Add a pref to enable the MobileViewportManager. r=tnikkel

Currently false by default, so no functional change in the default
configuration.

Differential Revision: https://phabricator.services.mozilla.com/D79223

layout/base/PresShell.cpp

@@ -10878,7 +10878,8 @@ bool UseMobileViewportManager(PresShell* aPresShell, Document* aDocument) {
       }
     }
   }
-  return nsLayoutUtils::ShouldHandleMetaViewport(aDocument) ||
+  return StaticPrefs::apz_mvm_force_enabled() ||
+         nsLayoutUtils::ShouldHandleMetaViewport(aDocument) ||
          nsLayoutUtils::AllowZoomingForDocument(aDocument);
 }
 

modules/libpref/init/StaticPrefList.yaml

@@ -494,6 +494,11 @@
   value: false
   mirror: always
 
+- name: apz.mvm.force-enabled
+  type: RelaxedAtomicBool
+  value: false
+  mirror: always
+
 - name: apz.one_touch_pinch.enabled
   type: RelaxedAtomicBool
   value: true