Roll external/skia 09a57b935..6e87eee2a (5 commits)

https://skia.googlesource.com/skia.git/+log/09a57b935..6e87eee2a

2018-01-18 reed@google.com add shadermaskfilter
2018-01-18 reed@google.com validate paint setters in readbuffer
2018-01-18 csmartdalton@google.com Extract a GrCCPathParser class that is decoupled from atlas ops
2018-01-18 ethannicholas@google.com SkSL now supports ternary lvalues
2018-01-18 reed@google.com detect fixed overflow in drawPoints

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Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
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Test: Presubmit checks will test this change.
Change-Id: I765da52d4180f50a84287e6d34131c6789b6d99d
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.