Populate VisibleSecurityState::displayed_mixed_content on iOS

On iOS, we know whether the page has displayed insecure content; this CL
propagates that information to SecurityStateModel::VisibleSecurityState.

As far as I can tell, the resulting value isn't used by anything on the iOS
port, so this CL will not result in any visible change at the current time.

Review URL: https://codereview.chromium.org/1578273002

Cr-Commit-Position: refs/heads/master@{#372051}

ios/chrome/browser/ssl/ios_chrome_security_state_model_client.cc

@@ -98,4 +98,8 @@ void IOSChromeSecurityStateModelClient::GetVisibleSecurityState(
   state->cert_status = ssl.cert_status;
   state->connection_status = ssl.connection_status;
   state->security_bits = ssl.security_bits;
+  state->displayed_mixed_content =
+      (ssl.content_status & web::SSLStatus::DISPLAYED_INSECURE_CONTENT)
+          ? true
+          : false;
 }