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Ensure that the SequenceLocalStorageMap and consequently objects still
stored in the sequence-local storage are destroyed while the owning
Sequence's SequenceToken is set for the current thread.
This makes sure that objects stored in the sequence-local storage which
expect to be destroyed on the same Sequence they've been created on do
not (D)CHECK in their destrutor.

Bug: 869272
Test: base_unittests && browser_tests --gtest_filter=*UserAffiliation*
Change-Id: I8c76b274a746a35c4ad038e7d8f04ea212ef1514
Reviewed-on: https://chromium-review.googlesource.com/1156510
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579834}