Added proper extension manifest parser for launcher_page.

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 fbf32291a92e65200a67c8efb3c168c04f6ee296..0a6d4f3091c88480c8a8f4f5d5f42a660593c1b7 100644
--- a/extensions/common/manifest_constants.cc
+++ b/extensions/common/manifest_constants.cc
@@ -464,6 +464,9 @@ const char kInvalidKioskOnly[] =
     "Invalid value for 'kiosk_only'.";
 const char kInvalidKioskOnlyButNotEnabled[] =
     "The 'kiosk_only' key is set, but 'kiosk_enabled' is not set.";
+const char kInvalidLauncherPage[] = "Invalid value for 'launcher_page'.";
+const char kInvalidLauncherPagePage[] =
+    "Invalid value for 'launcher_page.page'.";
 const char kInvalidLaunchContainer[] =
     "Invalid value for 'app.launch.container'.";
 const char kInvalidLaunchValue[] =
@@ -665,6 +668,8 @@ const char kInsecureContentSecurityPolicy[] =
     " see http://developer.chrome.com/extensions/contentSecurityPolicy.html";
 const char kKeyIsDeprecatedWithReplacement[] =
     "Key \"*\" is deprecated.  Key \"*\" should be used instead.";
+const char kLauncherPagePageRequired[] =
+    "The 'launcher_page.page' key is required.";
 const char kLaunchPathAndExtentAreExclusive[] =
     "The 'app.launch.local_path' and 'app.urls' keys cannot both be set.";
 const char kLaunchPathAndURLAreExclusive[] =