[iOS] Upstream switch and command for reader mode

ios/chrome/browser/chrome_switches.cc

 // Copyright 2015 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.
 
 #include "ios/chrome/browser/chrome_switches.h"
 
 namespace switches {
 
 // -----------------------------------------------------------------------------
 // When commenting your switch, please use the same voice as surrounding
 // comments. Imagine "This switch..." at the beginning of the phrase, and it'll
 // all work out.
 // -----------------------------------------------------------------------------
 
 // Disables the use of WKWebView instead of UIWebView.
 const char kDisableIOSWKWebView[] = "disable-wkwebview";
 
 // Enables Open from Clipboard.
 const char kEnableIOSOpenFromClipboard[] = "enable-ios-open-from-clipboard";
 
 // Enables the use of WKWebView instead of UIWebView.
 const char kEnableIOSWKWebView[] = "enable-wkwebview";
 
+// Enables context-sensitive reader mode button in the toolbar.
+const char kEnableReaderModeToolbarIcon[] = "enable-reader-mode-toolbar-icon";
+
 // Defines the value in MB of the memory wedge to insert at cold launch.
 const char kIOSMemoryWedgeSize[] = "ios-memory-wedge-size-mb";
 
 }  // namespace switches