Add Callback::IsCancelled

Callback::IsCancelled() checks if the Callback is cancelled by WeakPtr
invalidation.

Review-Url: https://codereview.chromium.org/2289703002
Cr-Commit-Position: refs/heads/master@{#417248}


CrOS-Libchrome-Original-Commit: 59aa6bb1162b31d2af31c4a235c67826687b6b28
5 files changed
tree: 0a5c87bd6468aace1bbc47ee900a3a719a31f068
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/