Add CommitSentToFirstSubresourceLoadStart trace event

This CL adds CommitSentToFirstSubresourceLoadStart trace event to
visualize the duration of
"PageLoad.Experimental.PageTiming.CommitSentToFirstSubresourceLoadStart"
UMA.

[Sample trace log]
https://ui.perfetto.dev/#!/?s=6bb79fed869257ea2fcf5223b09dc465782546b3badf1da8fbcc5624fa178825

Bug: 41482325
Change-Id: I1d8d8827650d73ed5e57e51c181b9ee8513e0b39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5306778


Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1262580}