Move --custom-launcher-page switch into app_list_switches.h.

ui/app_list/app_list_switches.cc

Index: ui/app_list/app_list_switches.cc
diff --git a/ui/app_list/app_list_switches.cc b/ui/app_list/app_list_switches.cc
index 37561fd693f7167967d3d2e7ab2dd13c1cfa8d35..ec2ea1cf8a3019c8917e46fd62f577042417367e 100644
--- a/ui/app_list/app_list_switches.cc
+++ b/ui/app_list/app_list_switches.cc
@@ -9,6 +9,10 @@
 namespace app_list {
 namespace switches {
 
+// Specifies the chrome-extension:// URL for the contents of an additional page
+// added to the experimental app launcher.
+const char kCustomLauncherPage[] = "custom-launcher-page";
+
 // If set, the app info context menu item is not available in the app list UI.
 const char kDisableAppInfo[] = "disable-app-list-app-info";