[ios/web] Always keep transient views on top.

In some cases, an transientContentView may be displayed when the
CRWWebControllerContainerView has no window.  When the
CRWWebControllerContainerView is put back in a window and the main
webViewContentView is restored, it will obscure the transientContentView.

Instead, always keep the transientContentView on top.

Bug: 949204
Change-Id: Ifa992620958ac3b7cfeb85d4451e2d13beee7810
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559894
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649297}

ios/web/web_state/ui/crw_web_controller_container_view.mm

@@ -134,6 +134,7 @@
   if (self.transientContentView) {
     if (!self.transientContentView.superview)
       [self addSubview:self.transientContentView];
+    [self bringSubviewToFront:self.transientContentView];
     self.transientContentView.frame = self.bounds;
   }
 }