Roll external/skia ea60b951d..b38bce94c (6 commits)

https://skia.googlesource.com/skia.git/+log/ea60b951d..b38bce94c

2018-08-21 angle-skia-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com Roll third_party/externals/angle2 7a4f6b8c7d5e..d710fee3861a (2 commits)
2018-08-21 herb@google.com Check for too small rect for path
2018-08-21 ethannicholas@google.com added support for sk_Dimensions to SkSL
2018-08-21 mtklein@google.com treat opaque dsts as src alpha type
2018-08-21 bsalomon@google.com Fix op chaining painter's order violation in GrRenderTargetOpList.
2018-08-21 herb@google.com Fix possible nullptr deref

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Change-Id: Ibfa89d1726d2197a3362d8acb40a469b62cd8343
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