[Extensions] Re-enable the TabsRelativeURLs test.

This test was disabled long ago as a result of a bad Webkit roll
and should have been re-enabled once that was resolved. I fixed
it and checked it for flakiness.

Change-Id: Ib8beac45cb3cda804d5652936e84faac0d1f4b4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414915Reviewed-by: Kelvin Jiang <kelvinjiang@chromium.org>
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810619}

chrome/browser/extensions/extension_tabs_apitest.cc

@@ -98,7 +98,7 @@ IN_PROC_BROWSER_TEST_F(ExtensionApiTabTest, TabEvents) {
   ASSERT_TRUE(RunExtensionSubtest("tabs/basics", "events.html")) << message_;
 }
 
-IN_PROC_BROWSER_TEST_F(ExtensionApiTabTest, DISABLED_TabRelativeURLs) {
+IN_PROC_BROWSER_TEST_F(ExtensionApiTabTest, TabRelativeURLs) {
   ASSERT_TRUE(RunExtensionSubtest("tabs/basics", "relative_urls.html"))
       << message_;
 }

chrome/test/data/extensions/api_test/tabs/basics/relative_urls.js

@@ -30,7 +30,7 @@ chrome.test.runTests([
     relativePageLoaded = chrome.test.callbackAdded();
 
     chrome.tabs.update(testTabId, {url: pageUrl("a")}, function(tab) {
-      chrome.test.assertEq(pageUrl("a"), tab.url);
+      chrome.test.assertEq(pageUrl("a"), tab.pendingUrl);
       chrome.tabs.update(tab.id, {url: "relative.html"}, function(tab) {
       });
     });