Cleanup PreferenceMACs on browser_tests startup

services/preferences/tracked/registry_hash_store_contents_win.h

Index: services/preferences/tracked/registry_hash_store_contents_win.h
diff --git a/services/preferences/tracked/registry_hash_store_contents_win.h b/services/preferences/tracked/registry_hash_store_contents_win.h
index 34496f97cec045cc4202a22a0ffc4a3d56c7742d..0b3b396205d60cda624442ace27e58a48357ce79 100644
--- a/services/preferences/tracked/registry_hash_store_contents_win.h
+++ b/services/preferences/tracked/registry_hash_store_contents_win.h
@@ -13,10 +13,15 @@
 class RegistryHashStoreContentsWin : public HashStoreContents {
  public:
   // Constructs a RegistryHashStoreContents which acts on a registry entry
-  // defined by |registry_path| and |store_key|.
+  // defined by |registry_path| and |store_key|. If |store_key| begins with
+  // base::ScopedTempDir::GetTempDirPrefix(), this RegistryHashStoreContentsWin
+  // will self Reset() on destruction to avoid proliferating keys in tests that
+  // create a profile in a ScopedTempDir (https://crbug.com/721245).
   explicit RegistryHashStoreContentsWin(const base::string16& registry_path,
                                         const base::string16& store_key);
 
+  ~RegistryHashStoreContentsWin() override;
+
   // HashStoreContents overrides:
   bool IsCopyable() const override;
   std::unique_ptr<HashStoreContents> MakeCopy() const override;
@@ -44,6 +49,7 @@ class RegistryHashStoreContentsWin : public HashStoreContents {
       const RegistryHashStoreContentsWin& other);
 
   const base::string16 preference_key_name_;
+  const bool reset_on_delete_;
 };
 
 #endif  // SERVICES_PREFERENCES_TRACKED_REGISTRY_HASH_STORE_CONTENTS_WIN_H_