Fix NullPointerException at chromeTabbedActivity#ShowOverview

crrev.com/c/2314557 removes the OverviewModeController wrapper in
ChromeTabbedActivity, and |mOverviewModeController| is null before
native loads which leads to a NullPointerException. This CL fixes the
crash by restoring the effective null checking in the
OverviewModeController wrapper.

Bug: 1112229
Change-Id: Ia62d9acdd267ed8c602fe61d85c2ecea8ac26747
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2337340Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Mei Liang <meiliang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796250}

chrome/android/java/src/org/chromium/chrome/browser/ChromeTabbedActivity.java

@@ -2015,7 +2015,7 @@ public class ChromeTabbedActivity extends ChromeActivity<ChromeActivityComponent
                 || state == OverviewModeState.SHOWING_START);
         if (mStartSurface != null) mStartSurface.getController().setOverviewState(state);
 
-        if (mOverviewModeController.overviewVisible()) return;
+        if (mOverviewModeController == null || mOverviewModeController.overviewVisible()) return;
 
         Tab currentTab = getActivityTab();
         // If we don't have a current tab, show the overview mode.
@@ -2048,7 +2048,8 @@ public class ChromeTabbedActivity extends ChromeActivity<ChromeActivityComponent
             getTabModelSelector().selectModel(incognito);
 
             if (TabUiFeatureUtilities.supportInstantStart(isTablet())
-                    || getTabModelSelector().isTabStateInitialized()) {
+                    || (getTabModelSelector().isTabStateInitialized()
+                            && isLayoutManagerCreated())) {
                 showOverview(OverviewModeState.SHOWING_HOMEPAGE);
             }
             return true;