Roll external/skia 5cab45e54..71713f6ea (4 commits)

https://skia.googlesource.com/skia.git/+log/5cab45e54..71713f6ea

2019-04-18 csmartdalton@google.com Swap front and back stencil sides when rendering bottom-left
2019-04-18 bsalomon@google.com Swap x/y radii for EllipticalRRectOp depending on matrix.
2019-04-18 halcanary@google.com SkQP: consolidate cut_release scripts
2019-04-18 recipe-mega-autoroller@chops-service-accounts.iam.gserviceaccount.com Roll recipe dependencies (trivial).

The AutoRoll server is located here: https://autoroll-internal.skia.org/r/android-master-autoroll

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
be CC'd on the roll, and stop the roller if necessary.

Test: Presubmit checks will test this change.
Change-Id: Ib77d3e6f5d8475f8270a7266e72e954437d0bb2a
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.