Roll src/third_party/skia/ cb4feeb7d..c51c18fd7 (3 commits).

https://skia.googlesource.com/skia.git/+log/cb4feeb7dbbe..c51c18fd7837

$ git log cb4feeb7d..c51c18fd7 --date=short --no-merges --format='%ad %ae %s'
2016-11-28 mtklein use fma() in accumulate
2016-11-28 reed don't support converting from alpha to non-alpha
2016-11-28 mtklein SkNx_abi is unused.

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=brianosman@google.com

Review-Url: https://codereview.chromium.org/2535013003
Cr-Commit-Position: refs/heads/master@{#434984}

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': 'cb4feeb7dbbe963dd966774afe743f254c70c79c',
+  'skia_revision': 'c51c18fd783737391e26541cbb5a72af2396ef7b',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.