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This CL moves code to load and manage test image files to a dedicated class.
Image metadata (such as image size, checksum,...) is stored in json files
accompanying the test files, so it doesn't need to be hardcoded in every test
anymore.

TEST=ran image_processor_test on nocturne

BUG=925223

Change-Id: I94d827be438e691f8c18307227bcd86a767e24a5
Reviewed-on: https://chromium-review.googlesource.com/c/1436278
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626933}