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This CL introduces a new pure virtual class, DriveChangeListLoader,
which we can specialize to load from the users default corpus or from
users team drives.

The first specialziation is DefaultCorpusChangeListLoader, which now
contains the existing change_list_loader, start_page_token_loader and
directory_loader objects.

Follow up work is to introduce a TeamDriveChangeListLoader which can
load change lists and retreive directories for specific team drives.

Bug: 715355

Change-Id: Ica88b2190c7d4abac6955784db7aada287f8be2c
Reviewed-on: https://chromium-review.googlesource.com/1064653
Commit-Queue: Stuart Langley <slangley@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560939}