drive: Implement Progressive Fast Fetch
Send entries to callbacks progressively.
Changes for ChangeListProcessor:
- RefreshDirectory is no longer responsible to update changestamp, DirectoryLoader will do it instead.
Changes for DirectoryLoader:
- Add a new method SendEntries() which is responsible to avoid sending the same entry for a callback twice.
- FeedFetcher is responsible to run RefreshDirectory() and SendEntries().
BUG=340931
TEST=unit_tests
Review URL: https://codereview.chromium.org/
194453002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@258923 0039d316-1c4b-4281-b951-d872f2087c98