Added 'launcher_page' field to extension manifest.json format.

extensions/common/manifest_constants.cc

Index: extensions/common/manifest_constants.cc
diff --git a/extensions/common/manifest_constants.cc b/extensions/common/manifest_constants.cc
index 95d6d8b43fcb07259cae7f7fd128a9db07b37d36..fbf32291a92e65200a67c8efb3c168c04f6ee296 100644
--- a/extensions/common/manifest_constants.cc
+++ b/extensions/common/manifest_constants.cc
@@ -68,6 +68,8 @@ const char kKioskMode[] = "kiosk_mode";
 const char kLanguage[] = "language";
 const char kLaunch[] = "app.launch";
 const char kLaunchContainer[] = "app.launch.container";
+const char kLauncherPage[] = "launcher_page";
+const char kLauncherPagePage[] = "launcher_page.page";
 const char kLaunchHeight[] = "app.launch.height";
 const char kLaunchLocalPath[] = "app.launch.local_path";
 const char kLaunchWebURL[] = "app.launch.web_url";