Roll external/skia da135c397..d74f2c897 (3 commits)

https://skia.googlesource.com/skia.git/+log/da135c397..d74f2c897

2018-08-17 z102.zhang@samsung.com After call 'va_start',some place not call 'va_end'
2018-08-17 z102.zhang@samsung.com BAD_COPY_PASTE in SkSLLayout.h
2018-08-17 kjlubick@google.com [PathKit] Add docs page

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Change-Id: Iba210dda6a637b37aec4811c828b25faf978c4d0
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