Bug 1443792 - Tighten the equivalence between a layer being scrollable and having an APZC. r=botond

If we can assume that a layer being composited has an APZC at index i if and
only if the frame metrics at index i is scrollable, then we can do the
transformations in the next patch without any change in functionality.

MozReview-Commit-ID: FRkvhwdd3nh

--HG--
extra : rebase_source : f1bee292305730079b3208e447330028c1a40727

gfx/layers/Layers.cpp

@@ -229,6 +229,8 @@ void
 Layer::SetAsyncPanZoomController(uint32_t aIndex, AsyncPanZoomController *controller)
 {
   MOZ_ASSERT(aIndex < GetScrollMetadataCount());
+  // We should never be setting an APZC on a non-scrollable layer
+  MOZ_ASSERT(!controller || GetFrameMetrics(aIndex).IsScrollable());
   mApzcs[aIndex] = controller;
 }
 

gfx/layers/Layers.h

@@ -1798,6 +1798,10 @@ public:
   // and can be used anytime.
   // A layer has an APZC at index aIndex only-if GetFrameMetrics(aIndex).IsScrollable();
   // attempting to get an APZC for a non-scrollable metrics will return null.
+  // The reverse is also true (that if GetFrameMetrics(aIndex).IsScrollable()
+  // is true, then the layer will have an APZC), although that only holds on
+  // the compositor-side layer tree, and only after the APZ code has had a chance
+  // to rebuild its internal hit-testing tree using the layer tree.
   // The aIndex for these functions must be less than GetScrollMetadataCount().
   void SetAsyncPanZoomController(uint32_t aIndex, AsyncPanZoomController *controller);
   AsyncPanZoomController* GetAsyncPanZoomController(uint32_t aIndex) const;