Comment by FateOfNations

Chrome extensions can access CPU and GPU statistics via a public API that any extension developer can use. Google Chrome comes bundled with a hidden "Hangouts Services" extension, which has permissions for *.google.com (that extension's ID is the 'nkeimhogjdpnpccoofpliimaahmaaome' referenced in the examples). That extension has a function that will return those stats when called from JavaScript running on a webpage. As a result, any JavaScript running on *.google.com can get the current CPU and GPU utilization.