Guard against crash in PreviewsLPNavThrottle

See bug for more details.

TBR=tbansal@chromium.org

Bug: 961934
Change-Id: I7cca3920fa8c8a1402a7c2189a5e8775de58bd30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1607182Reviewed-by: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658840}

chrome/browser/previews/previews_lite_page_navigation_throttle.cc

@@ -748,7 +748,8 @@ PreviewsLitePageNavigationThrottle::GetOrCreateServerLitePageInfo(
   const ChromeNavigationUIData* chrome_navigation_ui_data =
       static_cast<const ChromeNavigationUIData*>(
           navigation_handle->GetNavigationUIData());
-  info->page_id = chrome_navigation_ui_data->data_reduction_proxy_page_id();
+  if (chrome_navigation_ui_data)
+    info->page_id = chrome_navigation_ui_data->data_reduction_proxy_page_id();
   // The page id may not be set in some corner cases (like forward navigation),
   // so make sure it gets set here.
   if (info->page_id == 0U)