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This is possible now that we have infrastructure that glues an
IdentityTestEnvironment instance to a Profile, and it enables removing
the test's knowledge of SigninManager.

Bug: 896180
Change-Id: I074c0d2886561b264612ce2d34c5ca41b81de3be
Reviewed-on: https://chromium-review.googlesource.com/c/1286810Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600715}