Roll src/third_party/skia/ 941e7e2c9..18b4f981a (3 commits).

https://skia.googlesource.com/skia.git/+log/941e7e2c9567..18b4f981aeda

$ git log 941e7e2c9..18b4f981a --date=short --no-merges --format='%ad %ae %s'
2016-12-13 jvanverth Fix doc typo
2016-12-10 bsalomon Remove antialias axis from GrPathRendererChain::DrawType
2016-12-13 fmalita Avoid int64 overflow in SkClampRange

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, see:
http://www.chromium.org/developers/tree-sheriffs/sheriff-details-chromium#TOC-Failures-due-to-DEPS-rolls

CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel
TBR=msarett@google.com

Review-Url: https://codereview.chromium.org/2567243004
Cr-Commit-Position: refs/heads/master@{#438209}

DEPS

@@ -40,7 +40,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling Skia
   # and whatever else without interference from each other.
-  'skia_revision': '941e7e2c9567ab1d8a3b2d1b0e3db71ee5eb75c9',
+  'skia_revision': '18b4f981aeda10ab403208538bc732f31b56a094',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.