Fix -full fallback for diff updates (M56).

chrome/installer/mini_installer/regkey.h

 // Copyright (c) 2012 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.
 
 #ifndef CHROME_INSTALLER_MINI_INSTALLER_REGKEY_H_
 #define CHROME_INSTALLER_MINI_INSTALLER_REGKEY_H_
 
 #include <windows.h>
 #include <stddef.h>
 
@@ skipping 37 matching lines @@
                           const wchar_t *value_name, wchar_t *value,
                           size_t value_size);
 
  private:
   RegKey(const RegKey&);
   RegKey& operator=(const RegKey&);
 
   HKEY key_;
 };  // class RegKey
 
+// Initializes |key| with the desired |access| to |app_guid|'s Clients key.

[huangs, 2017/01/31 17:40:04]

I see this function in code search already; need to Sync?

Meanwhile, it looks like:
- configuration.cc is the only user of this.
- |app_guid| is only useful for GOOGLE_CHROME_BUILD.
- |app_guid| is always |google_update::kAppGuid|.

I'd recommend moving OpenClientKey() and OpenClientStateKey() to local
namespace{} inside configuration.cc, with hard-coded values. But this is beyond
scope of CL.

[grt (UTC plus 2), 2017/02/02 08:17:16]

Yeah, I don't really like this either. OpenClientStateKey is used outside of
configuration.cc, so I'm going to leave these as-is for now.

+// Returns ERROR_SUCCESS on success, or a Windows error code on failure.
+LONG OpenClientsKey(HKEY root_key,
+                    const wchar_t* app_guid,
+                    REGSAM access,
+                    RegKey* key);
 
-// Helper function to get the RegKey associated with the "client state" of
-// the given |app_guid|.
-bool OpenClientStateKey(HKEY root_key, const wchar_t* app_guid,
-                        REGSAM access, RegKey* key);
+// Initializes |key| with the desired |access| to |app_guid|'s ClientState key.
+// Returns ERROR_SUCCESS on success, or a Windows error code on failure.
+LONG OpenClientStateKey(HKEY root_key,
+                        const wchar_t* app_guid,
+                        REGSAM access,
+                        RegKey* key);
 
 }  // namespace mini_installer
 
 #endif  // CHROME_INSTALLER_MINI_INSTALLER_REGKEY_H_