Tab manager view should hide on press of back key

chrome/android/shell/java/src/org/chromium/chrome/shell/ChromeShellActivity.java

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.chrome.shell;
 
 import android.app.Activity;
 import android.content.Intent;
 import android.os.Bundle;
 import android.text.TextUtils;
@@ skipping 186 matching lines @@
 
     @Override
     protected void onSaveInstanceState(Bundle outState) {
         // TODO(dtrainor): Save/restore the tab state.
         if (mWindow != null) mWindow.saveInstanceState(outState);
     }
 
     @Override
     public boolean onKeyUp(int keyCode, KeyEvent event) {
         if (keyCode == KeyEvent.KEYCODE_BACK) {
+            if (mTabManager.isTabSwitcherVisible()) {
+                mTabManager.hideTabSwitcher();
+                return true;
+            }
             ChromeShellTab tab = getActiveTab();
             if (tab != null && tab.canGoBack()) {
                 tab.goBack();
                 return true;
             }
         }
-
         return super.onKeyUp(keyCode, event);
     }
 
     @Override
     protected void onNewIntent(Intent intent) {
         if (MemoryPressureListener.handleDebugIntent(this, intent.getAction())) return;
 
         String url = getUrlFromIntent(intent);
         if (!TextUtils.isEmpty(url)) {
             ChromeShellTab tab = getActiveTab();
@@ skipping 201 matching lines @@
     @VisibleForTesting
     public static void setActivityWindowAndroidFactory(ActivityWindowAndroidFactory factory) {
         sWindowAndroidFactory = factory;
     }
 
     @VisibleForTesting
     public static void setAppMenuHandlerFactory(AppMenuHandlerFactory factory) {
         sAppMenuHandlerFactory = factory;
     }
 }