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 06fa0a1ae6d68174bda946ee4ebb6598a2fa512e..4bb23b6d8da1610ed3efb702e757ec1efb30e1fb 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -830,9 +830,11 @@ 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();
+  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) {