[WIP] Add SharedMemoryTracker to dump base::SharedMemory usage

base/memory/discardable_shared_memory.h

Index: base/memory/discardable_shared_memory.h
diff --git a/base/memory/discardable_shared_memory.h b/base/memory/discardable_shared_memory.h
index ba3e16a465573965a4bd465b9e42178414ba8051..1cbafa65cd166aba02e8815b8c65f70a24680c0a 100644
--- a/base/memory/discardable_shared_memory.h
+++ b/base/memory/discardable_shared_memory.h
@@ -123,6 +123,8 @@ class BASE_EXPORT DiscardableSharedMemory {
   // It is safe to call Close repeatedly.
   void Close();
 
+  const SharedMemory& shared_memory() const { return shared_memory_; }
+
  private:
   // Virtual for tests.
   virtual Time Now() const;