Bug 950993 - Fix async scrolling of fixed layers within scrollable sub-layers. r=roc

Fixed layers are relative to their nearest scrollable ancestor, so when we
encounter a scrollable layer in AlignFixedAndStickyLayers, reset the transform
and sub-tree root to start from that scrollable layer.

This fixes async scrolling of the fixed header in the Firefox marketplace on
B2G.

gfx/layers/composite/AsyncCompositionManager.cpp

@@ -326,6 +326,16 @@ AsyncCompositionManager::AlignFixedAndStickyLayers(Layer* aLayer,
     return;
   }
 
+  // Fixed layers are relative to their nearest scrollable layer, so when we
+  // encounter a scrollable layer, reset the transform to that layer and remove
+  // the fixed margins.
+  if (aLayer->AsContainerLayer() &&
+      aLayer->AsContainerLayer()->GetFrameMetrics().IsScrollable() &&
+      aLayer != aTransformedSubtreeRoot) {
+    AlignFixedAndStickyLayers(aLayer, aLayer, aLayer->GetTransform(), LayerMargin(0, 0, 0, 0));
+    return;
+  }
+
   for (Layer* child = aLayer->GetFirstChild();
        child; child = child->GetNextSibling()) {
     AlignFixedAndStickyLayers(child, aTransformedSubtreeRoot,