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The in-progress cache is used to store metadata related to in-progress
downloads. The entry for the download is created upon the initiation of
the download and removed after the download is completed. This CL
continues the implementation of the in-progress cache by implementing
the read/write functionality in the following manner:

- making the in-progress cache an interface.
- initializing the cache during the creation of the download item,
halting the completion of the creation until the cache is initialized
(similar to the history database, which persists longer-term info).
- reading from the file during initialization and storing information
in an |entries| object.
- updating the |entries| object when adding/replacing/deleting and then
writing to the file using ImportantFileWriter.
- including unittests for this process.

Bug: 778425
Change-Id: I8f69dd51dd619dbc688c0f2b6a3b079a3b1164ec
Reviewed-on: https://chromium-review.googlesource.com/736907
Commit-Queue: Joy Ming <jming@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521158}