Adds DCHECK for identityManager in initializer

It should never be nil, except maybe in some tests.
In case of failing tests due to nil identityManager, the tests
should probably be fixed.

Change-Id: I86547615ce11c979c850a96dcc4ba5418396754d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1631670
Commit-Queue: Peter Lee <pkl@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670927}