[omnibox] Check `ShouldRunProvider()` for chrome://omnibox

The debug page would show results from providers that weren't running.
E.g., if you typed '@history query', you would see the most recent
search suggestions (matching '@history'). This was a bit confusing and
cluttered the debug page.

Also replaces some traditional `for(i)` loops with range `for(v : a)`.

Also re-orders autocomplete_controller.cc definitions to be consistent
with their header declarations.

Low-Coverage-Reason: TRIVIAL_CHANGE & debug code that doesn't affect users.
Change-Id: Id6144ed1ccf134534d4f0466bdb2a056428499a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5709095
Auto-Submit: manuk hovanesian <manukh@chromium.org>
Reviewed-by: Angela Yoeurng <yoangela@chromium.org>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1329097}