Adds a --deterministic-fetch flag to headless_shell

headless/public/util/generic_url_request_job_test.cc

Index: headless/public/util/generic_url_request_job_test.cc
diff --git a/headless/public/util/generic_url_request_job_test.cc b/headless/public/util/generic_url_request_job_test.cc
index 474b03eed775d507d674145c99001135bdf23da6..989b5f616381544fc822dd783dc42c3b58d11676 100644
--- a/headless/public/util/generic_url_request_job_test.cc
+++ b/headless/public/util/generic_url_request_job_test.cc
@@ -53,10 +53,12 @@ class MockFetcher : public URLFetcher {
   ~MockFetcher() override {}
 
   void StartFetch(const GURL& url,
+                  const std::string& method,
                   const net::HttpRequestHeaders& request_headers,
                   ResultListener* result_listener) override {
     // Record the request.
     fetch_request_->SetString("url", url.spec());
+    fetch_request_->SetString("method", method);
     std::unique_ptr<base::DictionaryValue> headers(new base::DictionaryValue);
     for (net::HttpRequestHeaders::Iterator it(request_headers); it.GetNext();) {
       headers->SetString(it.name(), it.value());
@@ -183,6 +185,7 @@ TEST_F(GenericURLRequestJobTest, BasicRequestParams) {
 
   std::string expected_request_json =
       "{\"url\": \"https://example.com/\","
+      " \"method\": \"GET\","
       " \"headers\": {"
       "   \"Accept\": \"text/plain\","
       "   \"Cookie\": \"\","
@@ -334,6 +337,7 @@ TEST_F(GenericURLRequestJobTest, RequestWithCookies) {
 
   std::string expected_request_json =
       "{\"url\": \"https://example.com/\","
+      " \"method\": \"GET\","
       " \"headers\": {"
       "   \"Cookie\": \"basic_cookie=1; secure_cookie=2; http_only_cookie=3\","
       "   \"Referer\": \"\""