Roll src/third_party/skia 12f8680a1e73..18531cde2ab7 (4 commits)

https://skia.googlesource.com/skia.git/+log/12f8680a1e73..18531cde2ab7

git log 12f8680a1e73..18531cde2ab7 --date=short --no-merges --format='%ad %ae %s'
2019-09-19 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/angle2 a7003d5fc306..bf4268a3963b (7 commits)
2019-09-19 skia-recreate-skps@skia-swarming-bots.iam.gserviceaccount.com Update Go deps
2019-09-19 skia-autoroll@skia-public.iam.gserviceaccount.com Roll ../src b1c1d284..b8149cbe (423 commits)
2019-09-19 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/swiftshader 61a2765940a7..e9b6f28fdd71 (3 commits)

Created with:
  gclient setdep -r src/third_party/skia@18531cde2ab7

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/skia-autoroll
Please CC brianosman@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

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

CQ_INCLUDE_TRYBOTS=luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux-blink-rel;luci.chromium.try:linux-chromeos-compile-dbg;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
TBR=brianosman@google.com

Bug: chromium:None,chromium:None,chromium:None
Change-Id: Ifc0a6fdfcd2e8ec3bdc45c0687efac189defda5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1812419Reviewed-by: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Commit-Queue: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#697943}

DEPS

@@ -162,7 +162,7 @@ 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': '12f8680a1e731a67129eb9dbd24536ffbbf9d9f6',
+  'skia_revision': '18531cde2ab757d82d3934fbac6c698e7b53cb3b',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.