Fix crash on login.

Fix crash on login.

When OwnerSettingsService::ReloadKeypair() is called, |private_key_| is
replaced with a new value. Thus a reference must be retained when we use
it in another thread.

BUG=chromium:711207
TEST=test_that samus tmp_LoginStressTest
     # https://chromium-review.googlesource.com/c/458138/

Review-Url: https://codereview.chromium.org/2818383002
Cr-Commit-Position: refs/heads/master@{#465096}
(cherry picked from commit 20138f3d0b1f72eb232363cac921dfc8c21812cf)

Review-Url: https://codereview.chromium.org/2828593003 .
Cr-Commit-Position: refs/branch-heads/3071@{#44}
Cr-Branched-From: a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed: https://chromium.googlesource.com/chromium/src/+/4c4a23513a0410c13828142b43b335983598acb8

[Shuhei Takahashi, 2017-04-19 04:16:20]

Description was changed from

==========
Fix crash on login.

When OwnerSettingsService::ReloadKeypair() is called, |private_key_| is
replaced with a new value. Thus a reference must be retained when we use
it in another thread.

BUG=chromium:711207
TEST=test_that samus tmp_LoginStressTest
     # https://chromium-review.googlesource.com/c/458138/

Review-Url: https://codereview.chromium.org/2818383002
Cr-Commit-Position: refs/heads/master@{#465096}
(cherry picked from commit 20138f3d0b1f72eb232363cac921dfc8c21812cf)
==========

to

==========
Fix crash on login.

When OwnerSettingsService::ReloadKeypair() is called, |private_key_| is
replaced with a new value. Thus a reference must be retained when we use
it in another thread.

BUG=chromium:711207
TEST=test_that samus tmp_LoginStressTest
     # https://chromium-review.googlesource.com/c/458138/

Review-Url: https://codereview.chromium.org/2818383002
Cr-Commit-Position: refs/heads/master@{#465096}
(cherry picked from commit 20138f3d0b1f72eb232363cac921dfc8c21812cf)

Review-Url: https://codereview.chromium.org/2828593003 .
Cr-Commit-Position: refs/branch-heads/3071@{#44}
Cr-Branched-From:
a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed:
https://chromium.googlesource.com/chromium/src/+/4c4a23513a0410c13828142b43b3...
==========

[Shuhei Takahashi, 2017-04-19 04:16:23]

Committed patchset #1 (id:1) manually as
4c4a23513a0410c13828142b43b335983598acb8.