Roll external/skia 3af31c0de..4fab53b37 (4 commits)

https://skia.googlesource.com/skia.git/+log/3af31c0de..4fab53b37

2018-04-23 bsalomon@google.com Fix starting location in GrRTC::drawOval when converting to GrShape
2018-04-23 bsalomon@google.com Analytic dashing of circles with single on/off intervals and butt caps.
2018-04-23 bsalomon@google.com Fix double counting of coverage in analytic round capped stroked circles.
2018-04-23 bsalomon@google.com Draw stroked circles with round caps analytically.

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Test: Presubmit checks will test this change.
Bug: 74608167
Change-Id: Idd4f0fe7fa9475dd5ca3c8a7d7297475a1bb1a0b
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