Improve error message on SequencedTaskRunnerHandle::Get().

TBR=danakj@chromium.org

Bug: Documentation update
Change-Id: I47be9a2f109001643c9910ae4db49e056baccc08
Reviewed-on: https://chromium-review.googlesource.com/580544Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488489}

base/threading/sequenced_task_runner_handle.cc

@@ -51,7 +51,11 @@ scoped_refptr<SequencedTaskRunner> SequencedTaskRunnerHandle::Get() {
   // sequenced task, return a SequencedTaskRunner for it.
   scoped_refptr<SequencedWorkerPool> pool =
       SequencedWorkerPool::GetWorkerPoolForCurrentThread();
-  DCHECK(pool);
+  // Note if you hit this: the problem isn't the lack of a |pool|, it's the lack
+  // of a sequenced context above. The |pool| is just the last desperate attempt
+  // at finding such a context from the deprecated SequencedWorkerPool.
+  DCHECK(pool) << "Error: This caller requires a sequenced context (i.e. the "
+                  "current task needs to run from a SequencedTaskRunner).";
   SequencedWorkerPool::SequenceToken sequence_token =
       SequencedWorkerPool::GetSequenceTokenForCurrentThread();
   DCHECK(sequence_token.IsValid());