[FPF] Slice PageLoad statistics by incognito

Create new histogram slices for incognito page loads of the existing
page load metrics. This is meant to be used for Fingerprinting
Protection only and should not affect the subresource filter
histograms that uses shared infrastructure.

TESTED=Locally: https://screenshot.googleplex.com/4AJhmMCgNRX6zQ5

Bug: 390396927
Change-Id: Ic6aaae930f77433dadeae8b2407b6e6eec54311a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6179931
Reviewed-by: Alex Turner <alexmt@chromium.org>
Reviewed-by: Zainab Rizvi <rizvis@google.com>
Reviewed-by: Martin Verde <thesalsa@google.com>
Commit-Queue: Patricia Rock <trishalfonso@google.com>
Cr-Commit-Position: refs/heads/main@{#1411720}