[Extensions] Remove EventRouterForwarder::DispatchEventToRenderers()

EventRouterForwarder::DispatchEventToRenderers() is used to dispatch an
event to extension listeners in a particular profile. The only utility
of it over using EventRouter directly is that it allows it to be called
from threads other than the UI thread.

However, in practice, this is only used in one place (the Proxy API) and
is always called from the UI thread.

Remove it from EventRouterForwarder and place it directly in the Proxy
API.

This CL should have no behavior change.

Bug: 360265324
Change-Id: I9b47b9d4e24b3a653ac2bd7d0da9701d68f82f27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5793901
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Reviewed-by: Justin Lulejian <jlulejian@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1343794}