service worker: Record fetch event milestones for main resource loading

This CL adds several UMAs to track time spend each stage of service
worker involved main resource loading. The purpose of these UMAs is
to make it easier to investigate performance bottleneck.

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: Ibb2433cafb090e66b450236ef2f5f1f4baa918bf
Reviewed-on: https://chromium-review.googlesource.com/c/1256384
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596903}
8 files changed