Remove deprecated SetString in WebUIDataSourceImpl

This removes the use of the deprecated method
DictionaryValue::SetString in WebUIDataSourceImpl.

It also fixes an unintended side effect where

WebUIDataSourceImpl::Add{Localized}String("foo.bar", value)
would create a nested 'foo' dictionary in the JS string
list, causing "foo.bar" to fail to localize when using
loadTimeData.getString('foo.bar').

Bug: None
Change-Id: I1da5f7da168acb681b93bbacccd361bea07800a7
Reviewed-on: https://chromium-review.googlesource.com/875158Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530542}

content/browser/webui/web_ui_data_source_impl.cc

@@ -115,22 +115,21 @@ WebUIDataSourceImpl::~WebUIDataSourceImpl() {
 void WebUIDataSourceImpl::AddString(const std::string& name,
                                     const base::string16& value) {
   // TODO(dschuyler): Share only one copy of these strings.
-  localized_strings_.SetString(name, value);
+  localized_strings_.SetKey(name, base::Value(value));
   replacements_[name] = base::UTF16ToUTF8(value);
 }
 
 void WebUIDataSourceImpl::AddString(const std::string& name,
                                     const std::string& value) {
-  localized_strings_.SetString(name, value);
+  localized_strings_.SetKey(name, base::Value(value));
   replacements_[name] = value;
 }
 
-void WebUIDataSourceImpl::AddLocalizedString(const std::string& name,
-                                             int ids) {
-  localized_strings_.SetString(
-      name, GetContentClient()->GetLocalizedString(ids));
-  replacements_[name] =
+void WebUIDataSourceImpl::AddLocalizedString(const std::string& name, int ids) {
+  std::string utf8_str =
       base::UTF16ToUTF8(GetContentClient()->GetLocalizedString(ids));
+  localized_strings_.SetKey(name, base::Value(utf8_str));
+  replacements_[name] = utf8_str;
 }
 
 void WebUIDataSourceImpl::AddLocalizedStrings(