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This change fixes a regression that was introduced in
https://chromium-review.googlesource.com/c/chromium/src/+/780279
Fixing the issue by switching sensitive use cases back to using
SkImage::readPixels for safely performing the unpremul conversion

BUG=826878

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ie4bc9ced4d7175ab308529f7dd7119f93378e34c
Reviewed-on: https://chromium-review.googlesource.com/1003320
Commit-Queue: Justin Novosad <junov@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550979}