Improve PictureLayerTiling::CoverageIterator to handle rounding more precisely

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 1a5bb8926b294b4b645a9c67e536d1b0c55e2e0b..bca3855e86af52fad4d0a6208aa68e0b27501791 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -835,9 +835,11 @@ void PictureLayerImpl::GetContentsResourceId(ResourceId* resource_id,
          bounds() == raster_source_->GetSize())
       << " bounds " << bounds().ToString() << " pile "
       << raster_source_->GetSize().ToString();
-  gfx::Rect content_rect(bounds());
+  float dest_scale = MaximumTilingContentsScale();

[enne (OOO), 2016/09/09 21:46:33]

I assume both this and the picture_layer_impl_unittest.cc changes are due to
this tripping DCHECKs when an incorrect destination scale is used?

[trchen, 2016/09/09 23:39:19]

Yes. And that DCHECK is important for correctness I think.

+  gfx::Rect content_rect =
+      gfx::ScaleToEnclosingRect(gfx::Rect(bounds()), dest_scale);
   PictureLayerTilingSet::CoverageIterator iter(
-      tilings_.get(), 1.f, content_rect, ideal_contents_scale_);
+      tilings_.get(), dest_scale, content_rect, ideal_contents_scale_);
 
   // Mask resource not ready yet.
   if (!iter || !*iter) {