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To migrate rule-based to Chromium C++, we need to have a separate code
path for the official XKB keyboard. The new code path needs to use Mojo
directly instead of sending extension messages.

Currently, IMF communicates to the extension via an "IME observer" which
takes native events like OnActivate and converts them to extension API
events.

To start off this migration, we create a new IME observer that basically
intercepts events from IMF and forwards them to the existing
extension-based IME observer. This allows us to migrate event-by-event
by routing a certain event directly to the IME service instead of
through the extension system.

In this CL, we hook into the OnActivate event to create a connection
to the IME service. All events are still forwarded to the old extension
system, so nothing should change.

In a followup CL, we will route OnReset directly to the IME service
as our first migration.

Bug: 1009903
Change-Id: Id4df50c8688474c76a498f5f2746f9ad2cbadabb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1847701
Auto-Submit: Darren Shen <shend@chromium.org>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Shu Chen <shuchen@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710192}