Roll Skia from 1313d1aa7a18 to 21bebc68fda6 (5 revisions)

https://skia.googlesource.com/skia.git/+log/1313d1aa7a18..21bebc68fda6

2020-09-09 skia-autoroll@skia-public.iam.gserviceaccount.com Roll dawn from 880013579381 to 0a1061da0c7f (14 revisions)
2020-09-09 johnstiles@google.com Return a 'made-changes' flag from control-flow optimization.
2020-09-09 reed@google.com remove unused (and bit-rotting) Lua support
2020-09-09 nifong@google.com Add a short description for concat and setmatrix saying whether it's a translate or something else
2020-09-09 cwallez@chromium.org Roll third_party/externals/spirv-cross/ 7778792ae..bdbef7b1f (19 commits)

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 reed@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: reed@google.com
Change-Id: Ibe82846287a44ba3bd38516afdda043f84045fba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401602Reviewed-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@{#805375}

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': '1313d1aa7a18d9c3f2dc6f9d04fc8c24bc91870c',
+  'skia_revision': '21bebc68fda61b4dedf7441942f3d6c160510a5f',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.