ServiceWorker: Clean up GetClientDocumentsCallback
Before this patch, GetClientDocumentsCallback counts the number of pending
GetClientInfoCallback to notice the completion of all dependent tasks.
Actually, this is not necessary because GetClientDocumentsCallback is owned
(only) by GetClientInfoCallback via scoped_refptr and the dtor of
GetClientDocumentsCallback is called when all tasks get completed.
After this patch, GetClientDocumentsCallback relies on the reference count
instead of counting pending tasks by itself and sends a completion message
on its dtor.
BUG=n/a
TEST=should pass all existing tests (just refactoring)
Review URL: https://codereview.chromium.org/
822323003
Cr-Commit-Position: refs/heads/master@{#309669}