[search-engine] Avoid syncing favicon-only updates

From manual testing, it is revealed that a significant portion of the
search engine sync updates only contains a favicon url change. This does
not seem effective given that favicons are fetched anyway upon
navigation. This CL implements logic to avoid sync a TURL change if only
the `favicon_url` field changes.

`ShouldCommitUpdateToAccount()` also contains condition to disregard a
turl if account data does not exist. This was initially done at
https://source.chromium.org/chromium/chromium/src/+/main:components/search_engines/template_url_service.cc;drc=3793eae8b53a0a10c9001592982cdf962af62a85;l=2121.
Additionally, we add `Sync.SearchEngine.FaviconOnlyUpdate` histogram to
track search engine updates with only favicon url changes.

Change-Id: I8074c4152d720227e1636cad8799ed994863e3e2
Bug: 417640596,415731522
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6545986
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Ankush Singh <ankushkush@google.com>
Auto-Submit: Ankush Singh <ankushkush@google.com>
Cr-Commit-Position: refs/heads/main@{#1463966}