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This fixes the issue when both Enrollment screen and Login screen trying
to load Gaia at the same time. SigninPartitionManager cancels one attempt
to start another one. This might break the enrollment flow when Chrome
can't extract oauth_code from the cookies partiton.

It's a bit hard to test as it's a race condition. I will add something
in a follow-up.

Bug: 1119411
Change-Id: I81c7031ae05b2882f4e86feac63e1aee9058c13e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2375426Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801818}