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EasyUnlockServiceSignin assumes that it is called in a signin context,
but in this particular codepath, ExistingUserController was calling
it in a multi-signin context. In that multi-signin context, the
EasyUnlockServiceSignin object being grabbed was already shut down,
with deleted members -- its use caused a segfault. This CL makes that
assumption clear and only uses EasyUnlockServiceSignin in a signin
context.

Also add check in EasyUnlockServiceSignIn::IsEnabled() that
|pref_manager| is valid.

Bug: 971202
Fix: 971202

Change-Id: Iccc86faaef74ac882bda34fbdde5e5c26394ceea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648674
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667304}