Add UMA for PurgeAndSuspend.

content/child/child_discardable_shared_memory_manager.h

Index: content/child/child_discardable_shared_memory_manager.h
diff --git a/content/child/child_discardable_shared_memory_manager.h b/content/child/child_discardable_shared_memory_manager.h
index 4a645fa0975a8cfc16a7b3b6129d7b606701cba9..9f83e1c19c887104db28e793d1263da9f7431e42 100644
--- a/content/child/child_discardable_shared_memory_manager.h
+++ b/content/child/child_discardable_shared_memory_manager.h
@@ -48,6 +48,13 @@ class CONTENT_EXPORT ChildDiscardableSharedMemoryManager
       const char* name,
       base::trace_event::ProcessMemoryDump* pmd) const;
 
+  struct Statistics {
+    size_t total_size;
+    size_t freelist_size;
+  };
+
+  Statistics GetStatistics() const;
+
  private:
   std::unique_ptr<base::DiscardableSharedMemory>
   AllocateLockedDiscardableSharedMemory(size_t size,