Roll external/skia af730f2b4..73329c8df (2 commits)

https://skia.googlesource.com/skia.git/+log/af730f2b4..73329c8df

2018-10-31 reed@google.com remove alignenum flag -- rely on clients to add it as needed
2018-10-31 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/angle2 193a284d34d2..d60e42f8e79e (1 commits)

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https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

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Change-Id: I12d60c186844f7596ab59c38e20f218afc5d17bb
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.