cc: Remove use of PassAs() and constructor-casting with scoped_ptr.

cc/test/fake_tile_manager.cc

Index: cc/test/fake_tile_manager.cc
diff --git a/cc/test/fake_tile_manager.cc b/cc/test/fake_tile_manager.cc
index d11d0fdb8ad4c9f4c638288cf59e0716b9c149df..6243ae2ef77543ced5e71e3dff89ca159af0018d 100644
--- a/cc/test/fake_tile_manager.cc
+++ b/cc/test/fake_tile_manager.cc
@@ -51,7 +51,7 @@ class FakeRasterizerImpl : public Rasterizer, public RasterizerTaskClient {
   // Overridden from RasterizerTaskClient:
   virtual scoped_ptr<RasterBuffer> AcquireBufferForRaster(
       const Resource* resource) OVERRIDE {
-    return scoped_ptr<RasterBuffer>();
+    return nullptr;
   }
   virtual void ReleaseBufferForRaster(
       scoped_ptr<RasterBuffer> buffer) OVERRIDE {}