Make proxy settings one atomic dictionary in the PrefStores such that modifications are atomic.

chrome/browser/prefs/proxy_prefs.h

Index: chrome/browser/prefs/proxy_prefs.h
diff --git a/chrome/browser/prefs/proxy_prefs.h b/chrome/browser/prefs/proxy_prefs.h
index da911d4e857db21329aaf8286ee7bd590d984008..a090a5c8daab67fe141aead1918c3d6b7dec2665 100644
--- a/chrome/browser/prefs/proxy_prefs.h
+++ b/chrome/browser/prefs/proxy_prefs.h
@@ -8,6 +8,8 @@
 
 #include <string>
 
+#include "base/values.h"
+
 namespace ProxyPrefs {
 
 // Possible types of specifying proxy settings. Do not change the order of
@@ -50,4 +52,40 @@ bool StringToProxyMode(const std::string& in_value,
 
 }  // namespace ProxyPrefs
 
+// Factory and wrapper for proxy preferences dictionaries that are stored
+// in the user preferences. The dictionary has the following structure:
+// {
+//   mode: string,
+//   server: string,
+//   pac_url: string,
+//   bypass_list: string
+// }
+// See proxy_prefs.cc for the structure of the respective strings.
+class ProxyPrefsDictionary {
+ public:
+  explicit ProxyPrefsDictionary(const DictionaryValue* dict);
+
+  bool GetMode(ProxyPrefs::ProxyMode* out) const;
+  bool GetPacUrl(std::string* out) const;
+  bool GetProxyServer(std::string* out) const;
+  bool GetBypassList(std::string* out) const;
+
+  static DictionaryValue* CreateDirect();
+  static DictionaryValue* CreateAutoDetect();
+  static DictionaryValue* CreatePacScript(const std::string& pac_url);
+  static DictionaryValue* CreateFixedServers(
+      const std::string& proxy_server,
+      const std::string& bypass_list);
+  static DictionaryValue* CreateSystem();
+ private:
+  static DictionaryValue* CreateDictionary(ProxyPrefs::ProxyMode mode,
+                                           const std::string& pac_url,
+                                           const std::string& proxy_server,
+                                           const std::string& bypass_list);
+
+  const DictionaryValue* dict_; // weak pointer
+
+  DISALLOW_COPY_AND_ASSIGN(ProxyPrefsDictionary);
+};
+
 #endif  // CHROME_BROWSER_PREFS_PROXY_PREFS_H_