Use a multimap to accumulate the values added to a group and
then add them to the Json group when it is finished.

Bug: angleproject:5853
Change-Id: Idbe82acc5f9a9671063f02bd1af039575aa52e87
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2824432
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>