[GWP] Add new trace trigger in OnStart()

This CL adds new trace trigger |gws-navigation-start| into
|GWSPageLoadMetricsObserver::OnStart()|.

Change-Id: I70321f64fdbbb81b74ac87abddedc84f63556f77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5599221
Reviewed-by: Minoru Chikamune <chikamune@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Keita Suzuki <suzukikeita@chromium.org>
Reviewed-by: Shunya Shishido <sisidovski@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313145}