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To make it easier to write fake DocumentLoaders for testing. Split the
implementation off into DocumentLoaderImpl.

Change-Id: I44f16c757b8a9be133936c41c231a552a7e05f0a
Reviewed-on: https://chromium-review.googlesource.com/1013998
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553816}