service worker: Record fetch event milestones for subresource loading

This is similar to https://crrev.com/c/1256384 but for subresource
loading.

Design doc:
https://docs.google.com/document/d/16K2X5k6hUq7lZSee-KXR9FUeSTEEypDXCjYbu_CvkT0/edit?usp=sharing

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Bug: 877972
Change-Id: Iba9c82b4de08862699eb1ba318f5710fe0264d86
Reviewed-on: https://chromium-review.googlesource.com/c/1264335
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600641}
4 files changed