Moved/renamed EscapeString from visual_studio_writer to XmlEscape in xml_element_writer.

tools/gn/xml_element_writer_unittest.cc

Index: tools/gn/xml_element_writer_unittest.cc
diff --git a/tools/gn/xml_element_writer_unittest.cc b/tools/gn/xml_element_writer_unittest.cc
index 93dfd47585130b5df9fa0e443519f37da7a33458..cc21c4bb66c62ae50804416b70f55aff3addccba 100644
--- a/tools/gn/xml_element_writer_unittest.cc
+++ b/tools/gn/xml_element_writer_unittest.cc
@@ -84,3 +84,10 @@ TEST(XmlElementWriter, StartContent) {
   }
   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);
+}