Migrate from MathUtil::inverse() to gfx::Transform::GetInverse()

cc/layer_tree_host_impl.cc

Index: cc/layer_tree_host_impl.cc
diff --git a/cc/layer_tree_host_impl.cc b/cc/layer_tree_host_impl.cc
index 55f6452ded8bc55344b72f71dc08478de00335a7..571f990ed845f10b9f25c8ee4f43171c2ebba6b3 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -491,8 +491,6 @@ static void appendQuadsToFillScreen(RenderPass* targetRenderPass, LayerImpl* roo
     // doesn't have any bounds and so can't generate this itself.
     // FIXME: Make the gutter quads generated by the solid color layer (make it smarter about generating quads to fill unoccluded areas).
 
-    DCHECK(rootLayer->screenSpaceTransform().IsInvertible());
-
     gfx::Rect rootTargetRect = rootLayer->renderSurface()->contentRect();
     float opacity = 1;
     SharedQuadState* sharedQuadState = quadCuller.useSharedQuadState(SharedQuadState::Create());
@@ -504,10 +502,13 @@ static void appendQuadsToFillScreen(RenderPass* targetRenderPass, LayerImpl* roo
                             opacity);
 
     AppendQuadsData appendQuadsData;
-    gfx::Transform transformToLayerSpace = MathUtil::inverse(rootLayer->screenSpaceTransform());
+
+    gfx::Transform transformToLayerSpace(gfx::Transform::kSkipInitialization);
+    bool didInvert = rootLayer->screenSpaceTransform().GetInverse(&transformToLayerSpace);
+    DCHECK(didInvert);
     for (Region::Iterator fillRects(fillRegion); fillRects.has_rect(); fillRects.next()) {
         // The root layer transform is composed of translations and scales only,
-        // no perspective, so mapping is sufficient.
+        // no perspective, so mapping is sufficient (as opposed to projecting).
         gfx::Rect layerRect = MathUtil::mapClippedRect(transformToLayerSpace, fillRects.rect());
         // Skip the quad culler and just append the quads directly to avoid
         // occlusion checks.
@@ -1277,7 +1278,11 @@ static gfx::Vector2dF scrollLayerWithViewportSpaceDelta(PinchZoomViewport* viewp
     // Layers with non-invertible screen space transforms should not have passed the scroll hit
     // test in the first place.
     DCHECK(layerImpl.screenSpaceTransform().IsInvertible());
-    gfx::Transform inverseScreenSpaceTransform = MathUtil::inverse(layerImpl.screenSpaceTransform());
+    gfx::Transform inverseScreenSpaceTransform(gfx::Transform::kSkipInitialization);
+    bool didInvert = layerImpl.screenSpaceTransform().GetInverse(&inverseScreenSpaceTransform);
+    // TODO: With the advent of impl-side crolling for non-root layers, we may
+    // need to explicitly handle uninvertible transforms here.
+    DCHECK(didInvert);
 
     gfx::PointF screenSpacePoint = gfx::ScalePoint(viewportPoint, scaleFromViewportToScreenSpace);