Describe AggregatedPreFCP and PreFCP differences in histograms.xml

Bug: 1374876
Change-Id: Id4b9d47bc6e6f5f2fd558027909ed52340dbddf0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3991411
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1064949}