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This makes it more clear that this policy applies only to lossy image
types (for now, just JPEG). This patch also makes the implementation
more generic in order to make the policy compatible with parsed
policy values and to pave the way for implementing a
lossless-enforcing version of the unoptimized-images policy as well.

This is a reland of this reverted CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1531354

Bug: 942659
Change-Id: I7928de8a7f3e66841ec3792a4152a3570a709ca3
TBR: iclelland@chromium.org,pdr@chromium.org,kenrb@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1541579
Commit-Queue: Paul Meyer <paulmeyer@chromium.org>
Reviewed-by: Paul Meyer <paulmeyer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644832}