DCHECK PrinterQuery runs on the IO thread.

Sprinkle DCHECK_CURRENTLY_ON(content::BrowserThread::IO) everywhere. Get
rid of |PrinterQuery::task_runner_| since it is always just the IO
thread.

Change-Id: I823a4b3a3359b3f66464561be142185cf48139f0
Reviewed-on: https://chromium-review.googlesource.com/c/1419159
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624360}
3 files changed