android: Null check SingleTabModeTabHelper

There are crash reports.

Bug: 769382
Change-Id: I4e406945605fa65193962048ad7e8961e8aa75c8
Reviewed-on: https://chromium-review.googlesource.com/826488Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524212}

chrome/browser/chrome_content_browser_client.cc

@@ -2579,7 +2579,8 @@ bool ChromeContentBrowserClient::CanCreateWindow(
 #if defined(OS_ANDROID)
   auto* single_tab_mode_helper =
       SingleTabModeTabHelper::FromWebContents(web_contents);
-  if (single_tab_mode_helper->block_all_new_windows()) {
+  if (single_tab_mode_helper &&
+      single_tab_mode_helper->block_all_new_windows()) {
     if (TabAndroid* tab_android = TabAndroid::FromWebContents(web_contents)) {
       tab_android->HandlePopupNavigation(&nav_params);
     }