Roll src/third_party/skia/ 2f145bff0..31981ecdc (8 commits)

https://skia.googlesource.com/skia.git/+log/2f145bff0b2e..31981ecdcb37

$ git log 2f145bff0..31981ecdc --date=short --no-merges --format='%ad %ae %s'
2017-07-28 ethannicholas fixed a vec2 -> float2 that got missed
2017-07-28 bsalomon Two small fixes from recent changes prepping for std::unique_ptr<GrFragmentProcessor>.
2017-07-28 ethannicholas renamed SkSL types in preparation for killing precision modifiers
2017-07-28 caryclark fix output header in bookmaker
2017-07-28 caryclark bookmaker generated site docs
2017-07-28 egdaniel Add Metal build bot
2017-07-28 bsalomon Add clone methods to unit test GrFragmentProcessor classes
2017-07-28 egdaniel Add placeholder GrMtlTypes in include/gpu/mtl

Created with:
  roll-dep src/third_party/skia


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;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
TBR=kjlubick@chromium.org

Change-Id: Ieee54c9ff81cf2aebfc927ddade9efceb4af9690
Reviewed-on: https://chromium-review.googlesource.com/592787Reviewed-by: Skia Deps Roller <skia-deps-roller@chromium.org>
Commit-Queue: Skia Deps Roller <skia-deps-roller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490635}

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': '2f145bff0b2e727c51ddd2f3eb2b90befb3ee118',
+  'skia_revision': '31981ecdcb37ef54fd34a0358fc6be471b43a47e',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.