bug 1286951 - Allow child histograms to be in processes.content as well as childPayloads[i]
bug 1286951 - Allow child histograms to be in processes.content as well as childPayloads[i]

When bug 1218576 lands, Firefox will start sending child histograms in
<payloadRoot>.processes.child.{keyedH|h}istograms
instead of
<payloadRoot>.childPayloads[i].{keyedH|h}istograms

Handle pings that have child histograms in either (or a mix of both) config.

Tested that current-format pings' child data is available via get_pings.

whd/python_moztelemetry