Roll external/skia acc75aa33..3af31c0de (2 commits)

https://skia.googlesource.com/skia.git/+log/acc75aa33..3af31c0de

2018-04-20 scroggo@google.com Reduce threads used for DNG decodes on Android
2018-04-19 scroggo@google.com Remove the limit on dng allocations

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Test: I3dc181297b6b51d5be100a44370194cba87e7a48
Bug: 78120086
Change-Id: I609f409d03b2725a84d0ae0fdc005ae21078e38c