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: Icb3b76ae44d44f8784d2d93d953790710eb2fb65
Reviewed-on: https://chromium-review.googlesource.com/c/1454365Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629189}

components/content_settings/core/browser/content_settings_pref_unittest.cc

@@ -109,8 +109,7 @@ TEST(ContentSettingsPref, CanonicalizationWhileReadingFromPrefs) {
   }
 
   TestingPrefServiceSimple prefs;
-  prefs.registry()->RegisterDictionaryPref(
-      kTestContentSettingPrefName, std::make_unique<base::DictionaryValue>());
+  prefs.registry()->RegisterDictionaryPref(kTestContentSettingPrefName);
   prefs.SetUserPref(kTestContentSettingPrefName,
                     std::move(original_pref_value));