[iOS] Fix WebStateDelegateTabHelperTest.GetJavaScriptDialogPresenter

The default cancellation handler is deleted after calling Cancel(),
so the observer needs to be removed before cancellation.

Bug: 1038514
Change-Id: I9a638b9ae7c6c44e4655fb5d80674285620c308d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032062Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737213}

ios/chrome/browser/overlays/default_overlay_request_cancel_handler.mm

@@ -43,16 +43,22 @@ void DefaultOverlayRequestCancelHandler::NavigationHelper::DidFinishNavigation(
   if (navigation_context->HasCommitted() &&
       !navigation_context->IsSameDocument()) {
     cancel_handler_->Cancel();
+    // The cancel handler is destroyed after Cancel(), so no code can be added
+    // after this call.
   }
 }
 
 void DefaultOverlayRequestCancelHandler::NavigationHelper::RenderProcessGone(
     web::WebState* web_state) {
   cancel_handler_->Cancel();
+  // The cancel handler is destroyed after Cancel(), so no code can be added
+  // after this call.
 }
 
 void DefaultOverlayRequestCancelHandler::NavigationHelper::WebStateDestroyed(
     web::WebState* web_state) {
-  cancel_handler_->Cancel();
   scoped_observer_.RemoveAll();
+  cancel_handler_->Cancel();
+  // The cancel handler is destroyed after Cancel(), so no code can be added
+  // after this call.
 }

ios/chrome/browser/web/web_state_delegate_tab_helper_unittest.mm

@@ -66,8 +66,7 @@ TEST_F(WebStateDelegateTabHelperTest, OnAuthRequired) {
 
 // Tests that GetJavaScriptDialogPresenter() returns an overlay-based JavaScript
 // dialog presenter.
-// TODO(crbug.com/1038514): Re-enable this test.
-TEST_F(WebStateDelegateTabHelperTest, DISABLED_GetJavaScriptDialogPresenter) {
+TEST_F(WebStateDelegateTabHelperTest, GetJavaScriptDialogPresenter) {
   // Verify that the delegate returns a non-null presenter.
   web::JavaScriptDialogPresenter* presenter =
       delegate()->GetJavaScriptDialogPresenter(web_state());