Roll external/skia f4c057af0..90019be56 (2 commits)

2019-05-01 Fixes for saveBehind/drawBehind
2019-05-01 Add secret draw-behind method

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Documentation for the AutoRoller is here:

If the roll is causing failures, please contact the current sheriff, who should
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Bug: 132354626
Bug: 129117085
Test: Presubmit checks will test this change.
Merged-In: I5b45b4581e20082a9875e6ee10ec51f24a7701db
Change-Id: Ifa794096cfcc6909256cb7f52627eaf9be0c10a8
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.