[Extensions cleanup] Update GetExtensionById() in //c/b/api

Use registry->enabled_extensions().GetByID(id) instead of
registry->GetExtensionById(id, ExtensionRegistry::ENABLED).

Bug: 40946544
Change-Id: Icb306d8352e3d82e154fa36016c50730712d9d3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5308003


Commit-Queue: Finnur Thorarinsson <finnur@chromium.org>
Auto-Submit: Emilia Paz <emiliapaz@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1263891}