[ios] Fix and reenable testChildTabOrdering EG test

The fix was copied from context_menu_egtest.mm which is to add a matcher
for context menu button.

Bug: 1106739
Change-Id: I1e89c3ccbc081aa0ff254a0a8bef535ed02054a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2304963
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Auto-Submit: Eugene But <eugenebut@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790046}

ios/chrome/browser/web/tab_order_egtest.mm

@@ -47,8 +47,7 @@ const char kLinksTestURL2Text[] = "arrived";
 @implementation TabOrderTestCase
 
 // Tests that new tabs are always inserted after their parent tab.
-// TODO(crbug.com/1106739): reenable this test.
-- (void)DISABLED_testChildTabOrdering {
+- (void)testChildTabOrdering {
   GREYAssertTrue(self.testServer->Start(), @"Test server failed to start.");
   const GURL URL1 = self.testServer->GetURL(kLinksTestURL1);
 
@@ -61,6 +60,8 @@ const char kLinksTestURL2Text[] = "arrived";
       performAction:chrome_test_util::LongPressElementForContextMenu(
                         [ElementSelector selectorWithElementID:kLinkSelectorID],
                         true /* menu should appear */)];
+  [[EarlGrey selectElementWithMatcher:OpenLinkInNewTabButton()]
+      assertWithMatcher:grey_notNil()];
   [[EarlGrey selectElementWithMatcher:OpenLinkInNewTabButton()]
       performAction:grey_tap()];
   [ChromeEarlGrey waitForMainTabCount:2U];