Roll Skia from 6de1e52d0b12 to 8a37fb2c605b (5 revisions) (#23836)

https://skia.googlesource.com/skia.git/+log/6de1e52d0b12..8a37fb2c605b

2021-01-21 tdenniston@google.com [svg] Implement SourceAlpha input for filter effects
2021-01-21 kjlubick@google.com [canvaskit] Include types in docker build
2021-01-21 tdenniston@google.com [svg] Several filter helper tweaks
2021-01-21 johnstiles@google.com Improve consistency of SPIR-V code generation.
2021-01-21 reed@google.com Update legacy shader context to take sampling

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/skia-flutter-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

DEPS

@@ -27,7 +27,7 @@ vars = {
   'skia_git': 'https://skia.googlesource.com',
   # OCMock is for testing only so there is no google clone
   'ocmock_git': 'https://github.com/erikdoe/ocmock.git',
-  'skia_revision': '6de1e52d0b126c52c41f350f42132965202deca2',
+  'skia_revision': '8a37fb2c605b492ab23f084e27b8f304a3dd50e6',
 
   # When updating the Dart revision, ensure that all entries that are
   # dependencies of Dart are also updated to match the entries in the

ci/licenses_golden/licenses_skia

@@ -1,4 +1,4 @@
-Signature: 55e4861ec77021661fbe5fc1e6132dd7
+Signature: 90a6c6caca7b8fc463375e3ca1a2bc58
 
 UNUSED LICENSES: