Preserve FrameSinkSourceMapping nodes that have path to root

cc/surfaces/surface_manager.cc

 // Copyright 2014 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/surfaces/surface_manager.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <queue>
@@ skipping 564 matching lines @@
       children[i] = children.back();
       children.resize(children.size() - 1);
       break;
     }
   }
   DCHECK(found_child);
 
   // The SurfaceFactoryClient and hierarchy can be registered/unregistered
   // in either order, so empty frame_sink_source_map entries need to be
   // checked when removing either clients or relationships.
-  if (!iter->second.has_children() && !clients_.count(parent_frame_sink_id)) {
+  if (!iter->second.has_children() && !clients_.count(parent_frame_sink_id) &&

[enne (OOO), 2017/03/17 17:00:38]

I'm not sure that this is enough.  The additional check is that it has a source
here, but what if you didn't register the begin frame source in your unit test
until after you attached C? It seems like C would not get a source at that
point.

If so would it make more sense for UnregisterFrameSinkHierarchy to remove the
*child* if it had no children or clients instead of the parent?  Or, maybe
there's some other solution here.

[enne (OOO), 2017/03/17 17:00:38]

I'm not sure that this is enough.  The additional check is that it has a source
here, but what if you didn't register the begin frame source in your unit test
until after you attached C? It seems like C would not get a source at that
point.

If so would it make more sense for UnregisterFrameSinkHierarchy to remove the
*child* if it had no children or clients instead of the parent?  Or, maybe
there's some other solution here.

[Fady Samuel, 2017/03/17 18:42:34]

As discussed offline, this isn't an issue. I've added an additional unit test to
verify this.

+      !iter->second.source) {
     frame_sink_source_map_.erase(iter);
     return;
   }
 
   // If the parent does not have a begin frame source, then disconnecting it
   // will not change any of its children.
   BeginFrameSource* parent_source = iter->second.source;
   if (!parent_source)
     return;
 
@@ skipping 92 matching lines @@
     std::vector<SurfaceId> children(iter->second.begin(), iter->second.end());
     std::sort(children.begin(), children.end());
 
     for (const SurfaceId& child_id : children)
       SurfaceReferencesToStringImpl(child_id, indent + "  ", str);
   }
 }
 #endif  // DCHECK_IS_ON()
 
 }  // namespace cc