Detach PasswordController in PasswordControllerTestSimple

With ARC, there is no garanty that PasswordController will be
deallocated before WebState.
Detach it before the end of the test.

Bug: None
Change-Id: Ib16320cba8dcb59fe79f65c633e063e29a35a614
Reviewed-on: https://chromium-review.googlesource.com/579373Reviewed-by: Vaclav Brozek <vabr@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488219}

ios/chrome/browser/passwords/password_controller_unittest.mm

@@ -1319,6 +1319,7 @@ TEST(PasswordControllerTestSimple, SaveOnNonHTMLLandingPage) {
   web_state.SetContentIsHTML(false);
   web_state.SetCurrentURL(GURL("https://example.com"));
   [passwordController webState:&web_state didLoadPageWithSuccess:YES];
+  [passwordController detach];
 }
 
 // Tests that an HTTP page without a password field does not update the SSL