Optimize map lookups in `SiteInstanceImpl` active document counts.

Refactor `IncrementActiveDocumentCount`, `DecrementActiveDocumentCount`, and `GetActiveDocumentCount` to reduce redundant map lookups. Use `operator[]`'s insertion behavior in `Increment`, and `find` with iterators in `Decrement` and `GetActiveDocumentCount` to perform only one lookup per operation.

Change-Id: I7395ca1d4560bde6563a2663fd746e43ae45a86c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7018144
Auto-Submit: Jeffrey Yu <yuje@google.com>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1526664}