Improve PictureLayerTiling::CoverageIterator to handle rounding more precisely

cc/layers/picture_layer_impl.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layers/picture_layer_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 817 matching lines @@
 }
 
 void PictureLayerImpl::GetContentsResourceId(ResourceId* resource_id,
                                              gfx::Size* resource_size) const {
   // The bounds and the pile size may differ if the pile wasn't updated (ie.
   // PictureLayer::Update didn't happen). In that case the pile will be empty.
   DCHECK(raster_source_->GetSize().IsEmpty() ||
          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) {
     *resource_id = 0;
     return;
   }
 
   // Masks only supported if they fit on exactly one tile.
   DCHECK(iter.geometry_rect() == content_rect)
       << "iter rect " << iter.geometry_rect().ToString() << " content rect "
@@ skipping 486 matching lines @@
 bool PictureLayerImpl::IsOnActiveOrPendingTree() const {
   return !layer_tree_impl()->IsRecycleTree();
 }
 
 bool PictureLayerImpl::HasValidTilePriorities() const {
   return IsOnActiveOrPendingTree() &&
          is_drawn_render_surface_layer_list_member();
 }
 
 }  // namespace cc