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This is needed to enable DestroyProfileOnBrowserClose by default in
automated tests (crrev.com/c/2503629).

GCMStoreImpl::Backend owns an sql::Database object, which may perform
IO when destroyed. Release it on the blocking task runner, so it
doesn't cause issues when the DestroyProfileOnBrowserClose flag is on.

Bug: 88586
Change-Id: I657ae488c6a55c1608bf45466ea6a3bc0153786f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2511250
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826278}