Roll Skia from 1b1c73ad500b to 8a9da73687e3 (4 revisions)

https://skia.googlesource.com/skia.git/+log/1b1c73ad500b..8a9da73687e3

2021-01-20 johnstiles@google.com Fix overzealous optimization of short-circuits.
2021-01-20 fmalita@chromium.org [svg] Fix handling of root element position attributes
2021-01-20 johnstiles@google.com Optimize (boolExpr == true) and (boolExpr != false) into boolExpr.
2021-01-20 brianosman@google.com Add ExternalFunction support to SkSL-to-SkVM

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 scroggo@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: scroggo@google.com
Change-Id: I159c236bdb828e124fae600ca5c5508d42487da0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2640884Reviewed-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@{#845375}

DEPS

@@ -199,7 +199,7 @@ 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': '1b1c73ad500b25aa16ec333a175796514a77978c',
+  'skia_revision': '8a9da73687e33a7fb97731aacdd2bbdd8cfd1f6b',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.