GitLab

- Re-post callbacks only if they are called synchronously
- Always return a valid operation ID (even if it's finished immediately)
- Fix cancel callbacks ordering

BUG=279288
TEST=existing tests
TBR=hidehiko@chromium.org, tzik@chromium.org

Review URL: https://codereview.chromium.org/23898002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221384 0039d316-1c4b-4281-b951-d872f2087c98