Add CSI metrics to GWS PLMO

This CL adds some of new metrics calculated with the timings which we
already record as UMAs in GWSPageLoadMetricsObserver.

Specifically, we'd like to add following:

- PageLoad.Clients.GoogleSearch.CSI.HeadChunkContentTime
- PageLoad.Clients.GoogleSearch.CSI.SearchContentTime
- PageLoad.Clients.GoogleSearch.CSI.TimeBetweenHCTAndSCT

HeadChunkContentTime is useful to know how long time it takes to process
the header chunk. SearchContentTime and TimeBetweenHCTAndSCT are useful
to know how long the browser has a gap time before processing the body
chunk.

Bug: 361008592
Change-Id: I04cb7a050cb4ae865e2268a9242a8fccc329244e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5799922
Reviewed-by: Minoru Chikamune <chikamune@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Shunya Shishido <sisidovski@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1345258}