Moved/renamed EscapeString from visual_studio_writer to XmlEscape in xml_element_writer.

tools/gn/xml_element_writer_unittest.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/xml_element_writer.h"
 
 #include <sstream>
 
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 66 matching lines @@
 }
 
 TEST(XmlElementWriter, StartContent) {
   std::ostringstream out;
   {
     XmlElementWriter writer(out, "foo", XmlAttributes("bar", "baz"));
     writer.StartContent(false) << "Hello world!";
   }
   EXPECT_EQ("<foo bar=\"baz\">Hello world!</foo>\n", out.str());
 }
+
+TEST(XmlElementWriter, TestXmlEscape) {
+  std::string input = "\r \n \t & < > \"";
+  std::string output = XmlEscape(input);
+  std::string expected = "&#13; &#10; &#9; &amp; &lt; &gt; &quot;";
+  EXPECT_EQ(expected, output);
+}