cc: Make the PictureLayerImpl raster source null until commit.

cc/layers/picture_layer_impl.cc

Index: cc/layers/picture_layer_impl.cc
diff --git a/cc/layers/picture_layer_impl.cc b/cc/layers/picture_layer_impl.cc
index a6792aa42aea393a77081062663194124817020b..a04549a19b340095349aacdda6eaa3e8b459669a 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -69,8 +69,6 @@ PictureLayerImpl::PictureLayerImpl(LayerTreeImpl* tree_impl,
     : LayerImpl(tree_impl, id),
       twin_layer_(nullptr),
       tilings_(CreatePictureLayerTilingSet()),
-      // TODO(danakj): Can this be null to start?
-      raster_source_(PicturePileImpl::Create()),
       ideal_page_scale_(0.f),
       ideal_device_scale_(0.f),
       ideal_source_scale_(0.f),
@@ -580,7 +578,9 @@ void PictureLayerImpl::UpdateRasterSource(
       << " bounds " << bounds().ToString() << " pile "
       << raster_source->GetSize().ToString();
 
-  bool could_have_tilings = CanHaveTilings();
+  // The |raster_source_| is initially null, so have to check for that for the
+  // first frame.
+  bool could_have_tilings = raster_source_.get() && CanHaveTilings();

[vmpstr, 2014/12/15 22:16:25]

nit: It would be cleaner, albeit slower, if this check was a part of
CanHaveTilings. I think this is fine, but give it a thought :P

[danakj, 2014/12/15 22:22:31]

I think I prefer it here, I want to be clear that code shouldn't be null
checking this in general, if you want to use the raster source and it's not
there you're doing something wrong, which I hope this comment and single
placement conveys.

   raster_source_.swap(raster_source);
 
   // The |new_invalidation| must be cleared before updating tilings since they