[omnibox] Fix broken zero-prefix suggestion precision/recall/usage.

This CL updates the way in which we record the session data associated
with zero-prefix suggestions to ensure that the precision/recall/usage
metrics emitted by the client properly represent non-trivial zero-prefix
suggestions in the Omnibox.

More specifically, this CL more closely associates the SessionData
struct definition with the AutocompleteMatch class and also introduces
an early-exit condition so that we no longer log any metrics for the
edge-case where a user selects the default (trivial) zero-prefix
suggestion in the Omnibox.

Bug: 421244751
Change-Id: I389f26442fff86d7ed6653558bcf5949964a6f83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6614182
Commit-Queue: Khalid Peer <khalidpeer@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1469667}