[Extensions] Re-enabled the Service Worker WebRequest test.

After testing 1000 iterations on the trybots, there were no failures.
The assumption is that recent work the the ServiceWorkerTaskQueue has
fixed the flaky behavior.

Bug: 997686
Change-Id: I836b23d13eb0994c3a6e3fbf319b95106bc22675
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929435Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719748}

chrome/browser/extensions/service_worker_apitest.cc

@@ -400,13 +400,7 @@ IN_PROC_BROWSER_TEST_F(ServiceWorkerBasedBackgroundTest, TabsExecuteScript) {
 }
 
 // Tests chrome.webRequest APIs.
-// Times out on Mac/Win only.  https://crbug.com/997686
-#if defined(OS_WIN) || defined(OS_MACOSX)
-#define MAYBE_WebRequest DISABLED_WebRequest
-#else
-#define MAYBE_WebRequest WebRequest
-#endif
-IN_PROC_BROWSER_TEST_F(ServiceWorkerBasedBackgroundTest, MAYBE_WebRequest) {
+IN_PROC_BROWSER_TEST_F(ServiceWorkerBasedBackgroundTest, WebRequest) {
   ASSERT_TRUE(
       RunExtensionTest("service_worker/worker_based_background/web_request"))
       << message_;