[ZPS] Records HTTP cache validity metric for zero-suggest responses

Adds a new feature param for omnibox::kZeroSuggestPrefetching
used to place users in counterfactual groups. Counterfactual
groups issue a follow-up non-cacheable request if the response
is loaded from the HTTP cache. The new response is compared
against the original cached response to determine HTTP cache
validity which is recorded as a new entry for the following
histograms:

Omnibox.ZeroSuggestRequests.Prefetch
Omnibox.ZeroSuggestRequests.NonPrefetch

Bug: 1262373
Change-Id: I1e106e2fb4cd89ac545d6ad6fb2e0ee605662a49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3343634
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: Mohamad Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#952525}