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Skia's default serialization behavior is to encode "bare" images via PNG.
For these utilities, we can just rely on that (except in the case of the
command-line option).

Bug: 
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: I439786209a59d5604a53d362e64703af430a8056
Reviewed-on: https://chromium-review.googlesource.com/824443Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Commit-Queue: Mike Reed <reed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523826}