[Compose] Proactive suggestion obeys contentsuggestions attribute.

See https://html.spec.whatwg.org/multipage/interaction.html#writing-suggestions for spec.

Fixed: 333928628
Change-Id: I85eb4d65103d6a981ce8b08bb7e501448c1531c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5472392
Commit-Queue: Christoph Schwering <schwering@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Christoph Schwering <schwering@google.com>
Auto-Submit: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1301970}