[Extensions] Rework inline installation observation

chrome/common/extensions/webstore_install_result.h

Index: chrome/common/extensions/webstore_install_result.h
diff --git a/chrome/common/extensions/webstore_install_result.h b/chrome/common/extensions/webstore_install_result.h
index 1378daee8385128ed50f20100e14f09a1ec74e0f..74d8184856782a862f2ae9a45805a044b057bf0e 100644
--- a/chrome/common/extensions/webstore_install_result.h
+++ b/chrome/common/extensions/webstore_install_result.h
@@ -9,6 +9,14 @@ namespace extensions {
 
 namespace webstore_install {
 
+extern const char kInvalidWebstoreItemId[];
+extern const char kWebstoreRequestError[];
+extern const char kInvalidWebstoreResponseError[];
+extern const char kInvalidManifestError[];
+extern const char kUserCancelledError[];
+extern const char kExtensionIsBlacklisted[];
+extern const char kInstallInProgressError[];
+
 // Result codes returned by WebstoreStandaloneInstaller and its subclasses.
 enum Result {
   // Successful operation.