Roll Skia from 43f49b4d52a4 to 939cc7a63059 (3 revisions)

https://skia.googlesource.com/skia.git/+log/43f49b4d52a4..939cc7a63059

2020-08-14 jvanverth@google.com Some fixes for Flutter memory issues.
2020-08-14 jvanverth@google.com Check for work completion before purging resources.
2020-08-14 johnstiles@google.com Fix SkSL constant propagation within nested casts.

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 fmalita@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/+doc/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
Cq-Do-Not-Cancel-Tryjobs: true
Bug: None
Tbr: fmalita@google.com
Change-Id: Id06fb1384e09ac8eddc37e922843bbc0e7a53111
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357560Reviewed-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@{#798395}

DEPS

@@ -195,7 +195,7 @@ 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': '43f49b4d52a42dd6d4ecdeca676f5589a43a849f',
+  'skia_revision': '939cc7a63059f1ba3a7a4be9bff07ba98c1b2c24',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.