Update the DevMode Chrome plugin's Makefile to build a zip file to upload to the Chrome web store.

Instead of building a crx file, we build a zip file containing the files to upload. Also, the Chrome web store doesn't allow the key or update URL to be set in the manifest.

Review at http://gwt-code-reviews.appspot.com/1858803

Review by: mdempsky@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11345 8db76d5a-ed1c-0410-87a9-c151d255dfc7