Add browser test for notification action icons.

content/public/common/content_switches.cc

Index: content/public/common/content_switches.cc
diff --git a/content/public/common/content_switches.cc b/content/public/common/content_switches.cc
index 7c4c219736897a3ced828fc65c557ce00f781073..befedc87557302ba6be25c02f188e35453691da7 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -403,6 +403,10 @@ const char kEnableNetworkInformation[]      = "enable-network-information";
 const char kEnableNonValidatingReloadOnRefreshContent[] =
     "enable-non-validating-reload-on-refresh-content";
 
+// Enables action button icons for the Web Notification API.
+const char kEnableNotificationActionIcons[] =
+    "enable-notification-action-icons";
+
 // Enables partial raster. Enabling this switch also enables the use of
 // persistent gpu memory buffers.
 const char kEnablePartialRaster[] = "enable-partial-raster";