Re-enable WebViewAPITest.TestWebRequestAPIWithHeaders in network service

I ran this a few hundred times with the ToTLinux build config, and
couldn't reproduce the flake. I'll monitor ToTLinux after this goes in
to see if the flake still happens.

Bug: 873792
Change-Id: Ibe427a800d0c3eaea5b887ffda140c0f556c92bb
Reviewed-on: https://chromium-review.googlesource.com/1188590Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586277}

extensions/browser/guest_view/web_view/web_view_apitest.cc

@@ -759,12 +759,6 @@ IN_PROC_BROWSER_TEST_F(WebViewAPITest, TestWebRequestAPI) {
 #define MAYBE_TestWebRequestAPIWithHeaders TestWebRequestAPIWithHeaders
 #endif
 IN_PROC_BROWSER_TEST_F(WebViewAPITest, MAYBE_TestWebRequestAPIWithHeaders) {
-#if defined(OS_LINUX)
-  if (base::FeatureList::IsEnabled(network::features::kNetworkService)) {
-    // Flaky on Linux, https://crbug.com/873792
-    return;
-  }
-#endif  // defined(OS_LINUX)
   std::string app_location = "web_view/apitest";
   StartTestServer(app_location);
   RunTest("testWebRequestAPIWithHeaders", app_location);