[Autofill] Fix metrics to measure the peak size for new form extraction

This CL fixes the Autofill.FormCacheSize metric to measure how the
peak size of the form cache changes also for the new form extraction
method FormCache::ModifiedExtractNewForms().

Bug: 1215333
Change-Id: Iceb664de03ab84b143ce1076c702fcff552b1b2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3116279


Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Eva Herencsárová <evih@google.com>
Cr-Commit-Position: refs/heads/main@{#915148}