Add '.js' to 'browser' field file names.

From some trial and error, it seems like files in the 'browser' package.json field need to include the file extension to work properly. Before changing this, I always got the output from `./src/node.js`; now it seems like it working.

Also, I removed the `var frames = global.frames` line from `./src/browser.js`, as there is no reference to a `frames` variable.