[ios] Disable testRequestMobileSiteGoBackToDesktop for SlimNav.

It appears tapping or swiping back to a different custom user agent broken after
iOS 12.2 with SlimNav enabled.

TBR: rohitrao
Bug: 990186
Change-Id: I5662684c405b030a176b75b25f00071473fb092a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1731146Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683427}

ios/chrome/browser/ui/popup_menu/request_desktop_mobile_site_egtest.mm

@@ -189,6 +189,10 @@ class UserAgentResponseProvider : public web::DataResponseProvider {
 // Tests that requesting mobile site of a page works and going back re-opens
 // desktop version of the page.
 - (void)testRequestMobileSiteGoBackToDesktop {
+  // TODO(crbug.com/990186): Re-enable this test.
+  if ([ChromeEarlGrey isSlimNavigationManagerEnabled])
+    EARL_GREY_TEST_DISABLED(@"Test disabled on SlimNavigationManager.");
+
   std::unique_ptr<web::DataResponseProvider> provider(
       new UserAgentResponseProvider());
   web::test::SetUpHttpServer(std::move(provider));