Convert --enable-md-history to a feature flag.

chrome/common/chrome_switches.h

Index: chrome/common/chrome_switches.h
diff --git a/chrome/common/chrome_switches.h b/chrome/common/chrome_switches.h
index c3fb63fc8cd03b57cd9f596c5763db8a54c91f38..3a40646d8a7bb3f08e34b012408578ced67c8c3d 100644
--- a/chrome/common/chrome_switches.h
+++ b/chrome/common/chrome_switches.h
@@ -8,6 +8,7 @@
 #ifndef CHROME_COMMON_CHROME_SWITCHES_H_
 #define CHROME_COMMON_CHROME_SWITCHES_H_
 
+#include "base/feature_list.h"
 #include "build/build_config.h"
 
 // Don't add more switch files here. This is linked into some places like the
@@ -122,7 +123,6 @@ extern const char kEnableExtensionActivityLogging[];
 extern const char kEnableExtensionActivityLogTesting[];
 extern const char kEnableFastUnload[];
 extern const char kEnableMaterialDesignExtensions[];
-extern const char kEnableMaterialDesignHistory[];
 extern const char kEnableMaterialDesignPolicyPage[];
 extern const char kEnableNaCl[];
 extern const char kEnableNativeNotifications[];
@@ -190,6 +190,7 @@ extern const char kLoadExtension[];
 extern const char kLocalNtpReload[];
 #endif
 extern const char kMakeDefaultBrowser[];
+extern const base::Feature kMaterialDesignHistoryFeature;
 extern const char kMediaCacheSize[];
 extern const char kMediaRouter[];
 extern const char kMessageLoopHistogrammer[];