[Cronet] Move the metrics back into their own struct.

http://crrev.com/c/1405287 added C generator support for structs-in-struct --
with this, we can move the metrics back into their own struct like the
Java API.

Bug: 879208
Change-Id: Ib5dada64b4a674a275d18eeb013e946480fc9435
Reviewed-on: https://chromium-review.googlesource.com/c/1407638
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Misha Efimov <mef@chromium.org>
Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623295}
5 files changed