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This change adds 16 bit decoding to PNGImageDecoder.
This change is a part of the bigger change to support 16 bit PNGs
in the new color managed canvas API
(chromium-review.googlesource.com/c/chromium/src/+/1079788).

BUG: 839034
Change-Id: I1bf92e1dd4fcbd79ffe7def33223fb5298649eab
Reviewed-on: https://chromium-review.googlesource.com/1148787Reviewed-by: Leon Scroggins <scroggo@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Mohammad Reza Zakerinasab <zakerinasab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583760}