This change makes the results processor to save the merged report
instead of just the last chunk of the report if the original report is
chunked when uploaded.

When the uploader does not provide a channel label, the processor will
now set it based on a new property in the report,
`run_info.browser_channel`, introduced in
web-platform-tests/wpt#12679 .

A new channel label, "beta", is also introduced in this commit, since we
now have beta browsers running on Taskcluster daily.