this allows merging two stats counter objects, will be used in a future
CL to merge statistics for multiple video layers.
Bug: webrtc:10365
Change-Id: Iee9c48b68dfd7ba29537c14fc5f4a7c1c333d145
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/131942
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Yves Gerey <yvesg@google.com>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27534}