Add heuristics to limit showing of new backspace UI bubble.

chrome/common/pref_names.h

 // Copyright (c) 2012 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.
 
 // Constants for the names of various preferences, for easier changing.
 
 #ifndef CHROME_COMMON_PREF_NAMES_H_
 #define CHROME_COMMON_PREF_NAMES_H_
 
 #include <stddef.h>
@@ skipping 328 matching lines @@
 extern const char kPluginsMetadata[];
 extern const char kPluginsResourceCacheUpdate[];
 #endif
 extern const char kDefaultBrowserLastDeclined[];
 extern const char kResetCheckDefaultBrowser[];
 extern const char kDefaultBrowserSettingEnabled[];
 #if defined(OS_MACOSX)
 extern const char kShowUpdatePromotionInfoBar[];
 #endif
 extern const char kUseCustomChromeFrame[];
+extern const char kBackShortcutBubbleShownCount[];
 #if defined(ENABLE_PLUGINS)
 extern const char kContentSettingsPluginWhitelist[];
 #endif
 extern const char kPartitionDefaultZoomLevel[];
 extern const char kPartitionPerHostZoomLevels[];
 
 #if !defined(OS_ANDROID)
 extern const char kPinnedTabs[];
 #endif
 
@@ skipping 447 matching lines @@
 #if defined(GOOGLE_CHROME_BUILD)
 extern const char kMediaRouterCloudServicesPrefSet[];
 extern const char kMediaRouterEnableCloudServices[];
 #endif  // defined(GOOGLE_CHROME_BUILD)
 extern const char kMediaRouterFirstRunFlowAcknowledged[];
 #endif
 
 }  // namespace prefs
 
 #endif  // CHROME_COMMON_PREF_NAMES_H_