Fix flakiness DocumentLoader#testCancelTask.

For testing, we use blockDocument and unblockDocument test mehtods to
control flow of DocumentLoader's background thread.
Previously testCancelTask may exit before the background thread goes
back from blocking, which causes InterruptedException.

Fixes: 28125289
Change-Id: Id03826733c5b6f1da66b9280838eb1d2897ed5fc