DCHECK tasks posted by TaskRunner::PostTask

This CL adds DCHECKs to detect null task posted to TaskRunner::PostTask
implementations, so that we can detect it earlier with meaningful stack
trace.

BUG=706854

Review-Url: https://codereview.chromium.org/2785943004
Cr-Commit-Position: refs/heads/master@{#461766}


CrOS-Libchrome-Original-Commit: c69769616ffc1d266a8c4af8be6b957e2f305da8
9 files changed
tree: 03fb879e4b2f3529679ff185732037ef098a78bb
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/