Re-enable OverlayPresentationContextImplTest.ContainedOverlayUI

This test was originally failing on iOS 12 device, but disabled for all
configurations. The test passes locally on available devices (iOS 13.5.1
and iOS 14).

Bug: 1060758
Change-Id: I14cab0b8aaff25c9da4728459ba0819c0b5a4e35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264852Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782038}

ios/chrome/browser/ui/overlays/overlay_presentation_context_impl_unittest.mm

@@ -287,8 +287,7 @@ TEST_F(OverlayPresentationContextImplTest, CanShowRequest) {
 }
 
 // Tests the presentation flow for contained overlay UI.
-// TODO(crbug.com/1060758): Re-enable after fixing failing test.
-TEST_F(OverlayPresentationContextImplTest, DISABLED_ContainedOverlayUI) {
+TEST_F(OverlayPresentationContextImplTest, ContainedOverlayUI) {
   std::unique_ptr<OverlayRequest> request =
       OverlayRequest::CreateWithConfig<TestContainedOverlay>();
   context_.PrepareToShowOverlayUI(request.get());