Flush all TaskScheduler<->MainLoop tasks in ~TestBrowserThreadBundle()

And update a bit of documentation on why some things are done a certain way.

BUG=724086

Review-Url: https://codereview.chromium.org/2893823003
Cr-Commit-Position: refs/heads/master@{#472864}


CrOS-Libchrome-Original-Commit: 13c37ee0b4babbdec65c0ea3c68c9d771999689c
1 file changed
tree: bb6355a603aaec4974698cf87fc4ac0162f17db5
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/