Enable testContextMenuOpenInNewTab.

The original failure reason is not included in the bug.
If the test was only  failing on device then it was not
disabled correctly.

The test has passed twice on the bots, so enabling it
to diagnose the problem.

Bug: 764691
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ie6787ca7124171c75d885322f77b124276723886
Reviewed-on: https://chromium-review.googlesource.com/892083Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532697}

ios/chrome/browser/context_menu/context_menu_egtest.mm

@@ -165,13 +165,6 @@ void SelectTabAtIndexInCurrentMode(NSUInteger index) {
 
 // Tests "Open in New Tab" on context menu.
 - (void)testContextMenuOpenInNewTab {
-  // TODO(crbug.com/764691): This test is flaky on iOS 11.  The bots retry
-  // failures, so this test sometimes appears green because it passes on the
-  // retry.
-  if (base::ios::IsRunningOnIOS11OrLater()) {
-    EARL_GREY_TEST_DISABLED(@"Test disabled on iOS 11.");
-  }
-
   // Set up test simple http server.
   std::map<GURL, std::string> responses;
   GURL initialURL = web::test::HttpServer::MakeUrl(kUrlInitialPage);