This DCHECK would have triggered in the test cases in the attached bug.

Bug: chromium:740591
Change-Id: Ib8e866fe60f5f4ee825e6772f68be768925ed792
Reviewed-on: https://chromium-review.googlesource.com/585401
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46891}