Add debug method to print surface references.

cc/surfaces/surface_manager.h

Index: cc/surfaces/surface_manager.h
diff --git a/cc/surfaces/surface_manager.h b/cc/surfaces/surface_manager.h
index b6885b81456a57a0ad6d6390afb37d5f71365e24..47651b77c07bd88d9ca148480d19b661cf1e1119 100644
--- a/cc/surfaces/surface_manager.h
+++ b/cc/surfaces/surface_manager.h
@@ -14,6 +14,7 @@
 #include <vector>
 
 #include "base/compiler_specific.h"
+#include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
@@ -26,6 +27,11 @@
 #include "cc/surfaces/surface_sequence.h"
 #include "cc/surfaces/surfaces_export.h"
 
+#if DCHECK_IS_ON()
+#include <iosfwd>
+#include <string>
+#endif
+
 namespace cc {
 class BeginFrameSource;
 class CompositorFrame;
@@ -43,6 +49,11 @@ class CC_SURFACES_EXPORT SurfaceManager
   explicit SurfaceManager(LifetimeType lifetime_type = LifetimeType::SEQUENCES);
   ~SurfaceManager() override;
 
+#if DCHECK_IS_ON()
+  // Returns a string representation of all reachable surface references.
+  std::string SurfaceReferencesToString();
+#endif
+
   void RegisterSurface(Surface* surface);
   void DeregisterSurface(const SurfaceId& surface_id);
 
@@ -126,6 +137,8 @@ class CC_SURFACES_EXPORT SurfaceManager
   }
 
  private:
+  using SurfaceIdSet = std::unordered_set<SurfaceId, SurfaceIdHash>;
+
   void RecursivelyAttachBeginFrameSource(const FrameSinkId& frame_sink_id,
                                          BeginFrameSource* source);
   void RecursivelyDetachBeginFrameSource(const FrameSinkId& frame_sink_id,
@@ -145,6 +158,13 @@ class CC_SURFACES_EXPORT SurfaceManager
   void RemoveSurfaceReferenceImpl(const SurfaceId& parent_id,
                                   const SurfaceId& child_id);
 
+#if DCHECK_IS_ON()
+  // Recursively prints surface references starting at |surface_id| to |str|.
+  void SurfaceReferencesToStringImpl(const SurfaceId& surface_id,
+                                     std::string indent,
+                                     std::stringstream* str);
+#endif
+
   // Use reference or sequence based lifetime management.
   LifetimeType lifetime_type_;
 
@@ -184,7 +204,6 @@ class CC_SURFACES_EXPORT SurfaceManager
   std::unordered_map<FrameSinkId, FrameSinkSourceMapping, FrameSinkIdHash>
       frame_sink_source_map_;
 
-  using SurfaceIdSet = std::unordered_set<SurfaceId, SurfaceIdHash>;
   // Tracks references from the child surface to parent surface. If there are
   // zero entries in the set for a SurfaceId then nothing is referencing the
   // surface and it can be garbage collected.