[Profile menu] Disable the command explicitly

This CL makes the IDC_SHOW_AVATAR_MENU command always explicitly enabled
or disabled. Even though each command is disabled by default, there's
one subtle difference of disabling it explicitly: it creates an instance
in the commands map and thus the instance does not need to get created
silently by a call to AddCommandObserver.

This fixes a bug that the command gets silently enabled by ToolbarView
observing it's state even though UpdateCommandEnabled() never gets
called.

Bug: 1224209
Change-Id: Icaf3a8b09f183cc415d4be2ee9ff9bf61199e2e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3038484
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#903034}