Roll src/third_party/skia 5c660e0cd868..4a6e84920793 (5 commits)

https://skia.googlesource.com/skia.git/+log/5c660e0cd868..4a6e84920793

git log 5c660e0cd868..4a6e84920793 --date=short --first-parent --format='%ad %ae %s'
2020-01-08 mtklein@google.com add add/sub/min/max w/ constant ops
2020-01-08 michaelludwig@google.com Reland "Avoid loop-dependent behavior in GrMemoryPoolBench"
2020-01-08 recipe-mega-autoroller@chops-service-accounts.iam.gserviceaccount.com Roll recipe dependencies (trivial).
2020-01-08 mtklein@google.com clamp to [0,1] before blending with normalized
2020-01-08 brianosman@google.com Expose the Input variable and Child name collections in SkRuntimeEffect

Created with:
  gclient setdep -r src/third_party/skia@4a6e84920793

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 jvanverth@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
Bug: None
Tbr: jvanverth@google.com
Change-Id: I98e9703e2620546b45209892709fdbe84e77e1be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1991787Reviewed-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@{#729547}

DEPS

@@ -181,7 +181,7 @@ 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': '5c660e0cd8682508c6e75f89145575a04bae3cdf',
+  'skia_revision': '4a6e84920793bd6f98c4a15f2eda90b7e4074a7b',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.