Enable WebNavigationApiTest.ServerRedirect

This test passes with iteration count of 100 locally. It should not be
flaky, but let's try and see.

Bug: 660288
Change-Id: I8f190be7f6b6639e048ea59f040ca0421c3b51bd
Reviewed-on: https://chromium-review.googlesource.com/600492Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491805}

chrome/browser/extensions/api/web_navigation/web_navigation_apitest.cc

@@ -399,8 +399,7 @@ IN_PROC_BROWSER_TEST_F(WebNavigationApiTest, ClientRedirect) {
       << message_;
 }
 
-// http://crbug.com/660288
-IN_PROC_BROWSER_TEST_F(WebNavigationApiTest, DISABLED_ServerRedirect) {
+IN_PROC_BROWSER_TEST_F(WebNavigationApiTest, ServerRedirect) {
   ASSERT_TRUE(StartEmbeddedTestServer());
   ASSERT_TRUE(RunExtensionTest("webnavigation/serverRedirect"))
       << message_;