Fix NPE during NewTabPageLayout destruction.

Null pointer crash is caused by CompositorViewHolder nulled out before onDestroy gets called in some cases.

Bug: 904245
Change-Id: I4a8c3347a2bf7f0af2f704bc2e33cadc4534adc7
Reviewed-on: https://chromium-review.googlesource.com/c/1334273
Commit-Queue: Cathy Li <chili@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607752}

chrome/android/java/src/org/chromium/chrome/browser/ntp/NewTabPageLayout.java

@@ -910,7 +910,10 @@ public class NewTabPageLayout extends LinearLayout implements TileGroup.Observer
 
     private void onDestroy() {
         VrModuleProvider.unregisterVrModeObserver(this);
-        if (mOverviewObserver != null) {
+        // Need to null-check compositor view holder and layout manager since they might've
+        // been cleared by now.
+        if (mOverviewObserver != null && mTab.getActivity().getCompositorViewHolder() != null
+                && mTab.getActivity().getCompositorViewHolder().getLayoutManager() != null) {
             ((LayoutManagerChrome) mTab.getActivity().getCompositorViewHolder().getLayoutManager())
                     .removeOverviewModeObserver(mOverviewObserver);
             mOverviewObserver = null;