Shutdown dependencies in IdentityManager destructor.

Rather than calling IdentityManager::Shutdown explicitly, perform the
Shutdown logic as part of ~IdentityManager(), so we don't need to
remember to call it.

This means that IdentityManager's dependencies will be shutdown later
than they were before this CL, but this shouldn't matter (tm) since the
dependencies are wholly owned by IdentityManager.

Bug: 945385
Change-Id: I061e78f90f9754716a8f537116dd55d2c769589b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578613
Commit-Queue: Lowell Manners <lowell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653554}
6 files changed