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This CL adds preferred apps class that contains preferred apps storage
struct, and add and find methods for the struct. Unit tests are also
included.

BUG=853604

Change-Id: I0c2c6c219720da5b56b1f1ed5e3818c760566487
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1864495
Commit-Queue: Maggie Cai <mxcai@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708490}