Fix Extensions.AppLaunchSource off-by-one error

extensions/common/api/app_runtime.idl

Index: extensions/common/api/app_runtime.idl
diff --git a/extensions/common/api/app_runtime.idl b/extensions/common/api/app_runtime.idl
index 0e251ff5b94c011577fe24bfae20d397be0900da..3ff0219752aae1e7e5f8f9978b18d135decc1da9 100644
--- a/extensions/common/api/app_runtime.idl
+++ b/extensions/common/api/app_runtime.idl
@@ -16,6 +16,11 @@ namespace app.runtime {
   };
 
   // Enumeration of app launch sources.
+  // This should be kept in sync with AppLaunchSource in
+  // extensions/common/constants.h, and GetLaunchSourceEnum() in
+  // extensions/browser/api/app_runtime/app_runtime_api.cc.
+  // Note the enumeration is used in UMA histogram so entries
+  // should not be re-ordered or removed.
   enum LaunchSource {
     untracked,
     app_launcher,