Mention ScopedTaskEnvironment as hint in post_task.cc instance DCHECK.

I often get asked what it means to hit this DCHECK and how to fix it.
Hint at typical solution directly in the error message.

The \n's attempt to isolate the hint between the CHECK and the Backtrace
in the log.

TBR=fdoray@chromium.org

Bug: 708584
Change-Id: I36417f1328313a15b274a2fcb26db9cc7418fe9e
Reviewed-on: https://chromium-review.googlesource.com/613686
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Francois Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#494096}
1 file changed