How to chrome web store validate manifest.json
Before uploading your extension to the webstore, it is really better to test by loading your extension as an "unpacked extension", which you can do by visiting chrome://extensions, expanding "Developer Mode" if it isn't already expanded, and then clicking "Load unpacked extension..."
Loading your extension will allow you to test it and discover issues, long before publishing it to test accounts or to other users. The Chrome extension developers' guide - manifest format documents what is permissible in the manifest. It is also necessary for the manifest to be standard JSON.
Also, I should point out that the Chrome webstore may disallow well-formatted code if it violates the terms (e.g. violating the branding guidelines or seeming to represent a website you don't own).
I found that Chrome store will not accept manifest.json including "// comment", but Chrome developer mode accepts.
I copy and paste from here, and "// Optional" is added to my manifest.json file.http://developer.chrome.com/extensions/manifest.html
Maybe Chrome developer mode should also reject the comments?