Enable WebNavigationApiTest.Download test.

This test was flaky due to a race condition in navigation code, which
has now been fixed. Let's try to enable it and see if the fix has helped
deflake this test.

Bug: 660288
Change-Id: Id5d74179b9719d97f712277d2fe2380b2235d74f
Reviewed-on: https://chromium-review.googlesource.com/1102569Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567688}

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

@@ -233,8 +233,7 @@ IN_PROC_BROWSER_TEST_F(WebNavigationApiTest, ServerRedirect) {
       << message_;
 }
 
-// https://crbug.com/660288
-IN_PROC_BROWSER_TEST_F(WebNavigationApiTest, DISABLED_Download) {
+IN_PROC_BROWSER_TEST_F(WebNavigationApiTest, Download) {
   ASSERT_TRUE(StartEmbeddedTestServer());
   content::DownloadManager* download_manager =
       content::BrowserContext::GetDownloadManager(browser()->profile());