Check if ChromeActivity.getSnackbarManager returns null in OfflineIndicator

Bug: 891084
Change-Id: Ie41ad671edcd94e43ff6f47e08839d736df170a6
Reviewed-on: https://chromium-review.googlesource.com/c/1257673Reviewed-by: Peter Williamson <petewil@chromium.org>
Commit-Queue: Jian Li <jianli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595967}

chrome/android/java/src/org/chromium/chrome/browser/offlinepages/indicator/OfflineIndicatorController.java

@@ -144,6 +144,7 @@ public class OfflineIndicatorController implements ConnectivityDetector.Observer
         if (activity == null) return;
         if (!(activity instanceof ChromeActivity)) return;
         ChromeActivity chromeActivity = (ChromeActivity) activity;
+        if (chromeActivity.getSnackbarManager() == null) return;
 
         if (isOnline) {
             hideOfflineIndicator(chromeActivity);