Bug 1150774 (Part 1) - Add helpers to simplify using typed units in ConfigureLayer. r=botond

layout/base/FrameLayerBuilder.h

@@ -86,17 +86,28 @@ struct ContainerLayerParameters {
     , mDisableSubpixelAntialiasingInDescendants(aParent.mDisableSubpixelAntialiasingInDescendants)
     , mInLowPrecisionDisplayPort(aParent.mInLowPrecisionDisplayPort)
   {}
+
   float mXScale, mYScale;
+
+  LayoutDeviceToLayerScale2D Scale() const {
+    return LayoutDeviceToLayerScale2D(mXScale, mYScale);
+  }
+
   /**
    * If non-null, the rectangle in which BuildContainerLayerFor stores the
    * visible rect of the layer, in the coordinate system of the created layer.
    */
   nsIntRect* mLayerContentsVisibleRect;
+
   /**
    * An offset to apply to all child layers created.
    */
   nsIntPoint mOffset;
 
+  LayerIntPoint Offset() const {
+    return LayerIntPoint::FromUntyped(mOffset);
+  }
+
   nscolor mBackgroundColor;
   bool mInTransformedSubtree;
   bool mInActiveTransformedSubtree;

layout/base/Units.h

@@ -309,6 +309,10 @@ struct LayerPixel {
     return nsIntRect(aRect.x, aRect.y, aRect.width, aRect.height);
   }
 
+  static nsIntPoint ToUntyped(const LayerIntPoint& aPoint) {
+    return nsIntPoint(aPoint.x, aPoint.y);
+  }
+
   static gfx::IntRect ToUnknown(const LayerIntRect& aRect) {
     return gfx::IntRect(aRect.x, aRect.y, aRect.width, aRect.height);
   }

layout/base/nsDisplayList.cpp

@@ -833,8 +833,7 @@ nsDisplayScrollLayer::ComputeFrameMetrics(nsIFrame* aForFrame,
   // all the pres shells from here up to the root, as well as any css-driven
   // resolution. We don't need to compute it as it's already stored in the
   // container parameters.
-  metrics.SetCumulativeResolution(LayoutDeviceToLayerScale2D(aContainerParameters.mXScale,
-                                                             aContainerParameters.mYScale));
+  metrics.SetCumulativeResolution(aContainerParameters.Scale());
 
   LayoutDeviceToScreenScale2D resolutionToScreen(
       presShell->GetCumulativeResolution()