cros: Fix crash caused by calling AppListView layout()

Fix a bug in which AppListView crashes when layout()
is called. This is because
|app_list_background_shield_mask_| may not be
initialized when layout() is called.

Bug: 882553
Change-Id: I582be13c4998c262de89cc9c46543bbdcb9c9801
Reviewed-on: https://chromium-review.googlesource.com/1220033Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Daniel Zhang <oxyflush@google.com>
Cr-Commit-Position: refs/heads/master@{#590506}

ash/app_list/views/app_list_view.cc

@@ -1157,7 +1157,8 @@ void AppListView::Layout() {
                                           -kAppListBackgroundRadius * 2);
   app_list_background_shield_->SetBoundsRect(app_list_background_shield_bounds);
   app_list_background_shield_->UpdateCornerRadius(kAppListBackgroundRadius);
-  if (is_background_blur_enabled_ && !IsHomeLauncherEnabledInTabletMode() &&
+  if (is_background_blur_enabled_ && app_list_background_shield_mask_ &&
+      !IsHomeLauncherEnabledInTabletMode() &&
       app_list_background_shield_->layer()->size() !=
           app_list_background_shield_mask_->layer()->size()) {
     // Update the blur mask for the |app_list_background_shield_| with same