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This method is unnecessary for two reasons.

1.  as IdentityManagerFactory constructor declares dependencies on
    other KeyedService's factories, this ensures they are built

2.  IdentityManagerFactory now only depends on SigninClientFactory
    that is created from EnsureBrowserStateKeyedServiceFactoriesBuilt

Bug: none
Change-Id: Icaa1b443519afbab90752d7869957458f5149d5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696966
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676834}