One integer specifying the version of the manifest file format your package requires. As of Chrome 18, developers should specify
2 (without quotes) to use the format as described by this document:
Consider manifest version 1 deprecated as of Chrome 18. Version 2 is not yet required, but we will, at some point in the not-too-distant future, stop supporting packages using deprecated manifest versions. Extensions, applications, and themes that aren't ready to make the jump to the new manifest version in Chrome 18 can either explicitly specify version
1, or leave the key off entirely.
The changes between version 1 and version 2 of the manifest file format are described in detail in the
manifest_version2 in Chrome 17 or lower is not recommended. If your extension needs to work in older versions of Chrome, stick with version 1 for the moment. We'll give you ample warning before version 1 stops working.