Currently this is feature is not available to the WebAPI interface. It is being considered for later versions of Reporter.
Workaround: Use the normaly User Interaface to download, archive, or email this same Report. One pre-defined report, amongst others, that will provide this information is the "Web Browsing Per Site" Report.
NOTE: For more information about the Webapi, see this article -000011882 , or the ONLINE Documentation.