Fix web_contents nullptr

Looks like this crash is happening right when the browser starts in some
weird corner case. This CL just exists early if web_contents() is null.

Bug: 876835
Change-Id: I6dc7eecbd42becb077d9156a28974264c4c90166
Reviewed-on: https://chromium-review.googlesource.com/1185898Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585377}

chrome/browser/previews/previews_ui_tab_helper.cc

@@ -59,6 +59,9 @@ void PreviewsUITabHelper::ShowUIElement(
     bool is_data_saver_user,
     bool is_reload,
     OnDismissPreviewsUICallback on_dismiss_callback) {
+  if (!web_contents())
+    return;
+
   // Retrieve PreviewsUIService* from |web_contents| if available.
   PreviewsService* previews_service = PreviewsServiceFactory::GetForProfile(
       Profile::FromBrowserContext(web_contents()->GetBrowserContext()));