merge b2g-inbound to mozilla-central

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "74ed51578aa6d69a994d88be8d97f3b136288cd7", 
+    "revision": "3fe38577e17209728a54b14624143dbf99ade2df", 
     "repo_path": "/integration/gaia-central"
 }

layout/base/nsDisplayList.cpp

@@ -2872,6 +2872,9 @@ RequiredLayerStateForChildrenInternal(nsDisplayListBuilder* aBuilder,
         state > result) {
       result = state;
     }
+    if (state == LAYER_ACTIVE_EMPTY && state > result) {
+      result = LAYER_ACTIVE_FORCE;
+    }
     if (state == LAYER_NONE) {
       nsDisplayList* list = i->GetSameCoordinateSystemChildren();
       if (list) {