Add this to your karma.conf.js:
coverageReporter: {
dir: 'coverage/',
subdir: 'Chrome',
file: 'index.html'
}
EDIT: To have a subdir name based on the name of the browser used for the test:
coverageReporter: {
dir: 'coverage',
subdir: function(browser) {
return browser.toLowerCase().split(/[ /-]/)[0];
},
file: 'index.html'
}