Add the about_page key to shared_module manifest

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 40f04bc7ab08b78eae5a42a53c7d81bff615e814..c8c41ef8350f5be26c0218d99d67639bd9947489 100644
--- a/extensions/common/manifest_constants.h
+++ b/extensions/common/manifest_constants.h
@@ -10,6 +10,7 @@ namespace extensions {
 // Keys used in JSON representation of extensions.
 namespace manifest_keys {
 
+extern const char kAboutPage[];
 extern const char kAllFrames[];
 extern const char kAltKey[];
 extern const char kApp[];
@@ -255,6 +256,8 @@ extern const char kChromeVersionTooLow[];
 extern const char kDevToolsExperimental[];
 extern const char kDisabledByPolicy[];
 extern const char kExperimentalFlagRequired[];
+extern const char kInvalidAboutPage[];
+extern const char kInvalidAboutPageExpectRelativePath[];
 extern const char kExpectString[];
 extern const char kInvalidAllFrames[];
 extern const char kInvalidBackground[];