Do not override User-Agent header added by WebCore

net/http/http_request_headers_unittest.cc

Index: net/http/http_request_headers_unittest.cc
diff --git a/net/http/http_request_headers_unittest.cc b/net/http/http_request_headers_unittest.cc
index f3abfbe2ca728f4ab5ab4b788f7e54ad9ada1fdf..384ceeb67adfc9d6152a3705cfd92fe10bc5b165 100644
--- a/net/http/http_request_headers_unittest.cc
+++ b/net/http/http_request_headers_unittest.cc
@@ -67,6 +67,14 @@ TEST(HttpRequestHeaders, SetEmptyHeader) {
   EXPECT_EQ("Foo: Bar\r\nBar:\r\n\r\n", headers.ToString());
 }
 
+TEST(HttpRequestHeaders, SetHeaderIfMissing) {
+  HttpRequestHeaders headers;
+  headers.SetHeaderIfMissing("Foo", "Bar");
+  EXPECT_EQ("Foo: Bar\r\n\r\n", headers.ToString());
+  headers.SetHeaderIfMissing("Foo", "Baz");
+  EXPECT_EQ("Foo: Bar\r\n\r\n", headers.ToString());
+}
+
 TEST(HttpRequestHeaders, RemoveHeader) {
   HttpRequestHeaders headers;
   headers.SetHeader("Foo", "bar");