Add an about:flag for enabling a reader mode toolbar icon.

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 47c03b11717b3de7d7f40d56b55f38e3ff70665c..82cffda54e1221049f1e48d6c8bfdacaab5a7bdb 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -552,6 +552,9 @@ const char kEnableQuicPortSelection[]       = "enable-quic-port-selection";
 const char kEnableQuicTimeBasedLossDetection[] =
     "enable-quic-time-based-loss-detection";
 
+// Enables context-sensitive reader mode button in the toolbar.
+const char kEnableReaderModeToolbarIcon[]      = "enable-reader-mode-toolbar-icon";
+
 // Enables save password prompt bubble.
 const char kEnableSavePasswordBubble[]      = "enable-save-password-bubble";