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: Ib121cd952c29d40d46ab65b02f1e305132131eab
Reviewed-on: https://chromium-review.googlesource.com/c/1454545Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629170}

chrome/browser/plugins/plugins_resource_service.cc

@@ -105,8 +105,7 @@ PluginsResourceService::~PluginsResourceService() {
 
 // static
 void PluginsResourceService::RegisterPrefs(PrefRegistrySimple* registry) {
-  registry->RegisterDictionaryPref(prefs::kPluginsMetadata,
-                                   std::make_unique<base::DictionaryValue>());
+  registry->RegisterDictionaryPref(prefs::kPluginsMetadata);
   registry->RegisterStringPref(prefs::kPluginsResourceCacheUpdate, "0");
 }