Added proper extension manifest parser for launcher_page.

extensions/common/manifest_constants.h

Index: extensions/common/manifest_constants.h
diff --git a/extensions/common/manifest_constants.h b/extensions/common/manifest_constants.h
index f9246cbf5e5c142b99d3d1bfcc0e9f8aef4c2791..e9b37ed53faae915631f16649588edf4173ee99d 100644
--- a/extensions/common/manifest_constants.h
+++ b/extensions/common/manifest_constants.h
@@ -354,6 +354,8 @@ extern const char kInvalidKeyBindingUnknownPlatform[];
 extern const char kInvalidKioskEnabled[];
 extern const char kInvalidKioskOnly[];
 extern const char kInvalidKioskOnlyButNotEnabled[];
+extern const char kInvalidLauncherPage[];
+extern const char kInvalidLauncherPagePage[];
 extern const char kInvalidLaunchContainer[];
 extern const char kInvalidLaunchValue[];
 extern const char kInvalidLaunchValueContainer[];
@@ -449,6 +451,7 @@ extern const char kInvalidWebURLs[];
 extern const char kInvalidZipHash[];
 extern const char kInsecureContentSecurityPolicy[];
 extern const char kKeyIsDeprecatedWithReplacement[];
+extern const char kLauncherPagePageRequired[];
 extern const char kLaunchPathAndExtentAreExclusive[];
 extern const char kLaunchPathAndURLAreExclusive[];
 extern const char kLaunchURLRequired[];