Roll external/skia e027e1947..f8a131dd4 (3 commits)

https://skia.googlesource.com/skia.git/+log/e027e1947..f8a131dd4

2018-03-13 bungeman@google.com Compile with GCC 7.2.0.
2018-03-13 fmalita@chromium.org Harden initial LinearGradient4f tiling
2018-03-13 benjaminwagner@google.com Add Win Clang x86 builds.

The AutoRoll server is located here: https://android-roll.skia.org

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
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Test: Presubmit checks will test this change.
Change-Id: I7235bf8cba82ba6e2b07cca3ca1250b23a7ad717
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.