Switch from callbacks to an observer for TestLauncher::LaunchChildGTestProcess.

Originally, LaunchChildGTestProcess took a single callback that was run
on the caller's thread when all processing was done for the child
process. Eventually, a second callback was added to give consumers a
chance to act immediately on process launch. This here change switches
from two callbacks to an observer interface in preparation for a third
observation point.

BUG=764415
R=sky@chromium.org,phajdan.jr@chromium.org

Change-Id: I38a8a99a78007d4598e650b85d9936beb1ef6a1e
Reviewed-on: https://chromium-review.googlesource.com/746682
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513089}

CrOS-Libchrome-Original-Commit: ced5430e46b1fa8cbcd75b10118ead4cff78acac
3 files changed
tree: d1f18e4e787a13de02dd973868c721172bf70abd
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/