Add a null check before attempting to record tab switch metric.

R=yusufo@chromium.org

Bug: 988095
Change-Id: Ie94e97fc7016f57ac212e402e29c07a208dfee74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1722985Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Matt Simmons <mattsimmons@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681932}

chrome/android/features/tab_ui/java/src/org/chromium/chrome/browser/tasks/tab_management/GridTabSwitcherMediator.java

@@ -309,6 +309,8 @@ class GridTabSwitcherMediator implements GridTabSwitcher.GridController,
      *               in TabModel.
      */
     private void recordUserSwitchedTab(Tab tab, int lastId) {
+        if (tab == null) return;
+
         Tab fromTab = TabModelUtils.getTabById(mTabModelSelector.getCurrentModel(), lastId);
         assert fromTab != null;