Remove uses of deprecated RegisterDictionaryPref

The override of RegisterDictionaryPref taking a std::unique_ptr<base::Value>
is deprecated in favor of the method taking the base::Value by value.

Convert client code to either not pass default value if the value is just
an empty dictionary (as there is an override doing this). If the dictionary
is not empty, convert the code to create and pass it by value.

This CL was uploaded by git cl split.

Bug: 646113
Change-Id: I3cc0af55c8101bf35defc79068a48d11f3c6ff73
Reviewed-on: https://chromium-review.googlesource.com/c/1454523
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629184}

components/proxy_config/pref_proxy_config_tracker_impl.cc

@@ -236,17 +236,15 @@ PrefProxyConfigTrackerImpl::GetEffectiveProxyConfig(
 
 // static
 void PrefProxyConfigTrackerImpl::RegisterPrefs(PrefRegistrySimple* registry) {
-  registry->RegisterDictionaryPref(
-      proxy_config::prefs::kProxy,
-      std::make_unique<base::Value>(ProxyConfigDictionary::CreateSystem()));
+  registry->RegisterDictionaryPref(proxy_config::prefs::kProxy,
+                                   ProxyConfigDictionary::CreateSystem());
 }
 
 // static
 void PrefProxyConfigTrackerImpl::RegisterProfilePrefs(
     PrefRegistrySimple* registry) {
-  registry->RegisterDictionaryPref(
-      proxy_config::prefs::kProxy,
-      std::make_unique<base::Value>(ProxyConfigDictionary::CreateSystem()));
+  registry->RegisterDictionaryPref(proxy_config::prefs::kProxy,
+                                   ProxyConfigDictionary::CreateSystem());
   registry->RegisterBooleanPref(proxy_config::prefs::kUseSharedProxies, false);
 }