Create Inactive Tabs classes when feature is available

This CL makes sure to create the always-present inactive tabs
coordinator and inactive tabs button mediator when the feature is
available (Finch) but disabled by the user. This way, when the user
reenables it in Settings, the classes are available.

Fixed: 1440737
Change-Id: I271e209634b7ed37e3771badac265da79f00a9f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4484637
Commit-Queue: Louis Romero <lpromero@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1137042}