Roll Skia from 78f3a9a4587c to 6920730bc2e8 (1 revision) (flutter/engine#42441)

https://skia.googlesource.com/skia.git/+log/78f3a9a4587c..6920730bc2e8

2023-05-31 skia-autoroll@skia-public.iam.gserviceaccount.com Roll vulkan-deps from ea3ba34f308e to baa9940928e0 (3 revisions)

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 brianosman@google.com,jonahwilliams@google.com,rmistry@google.com,robertphillips@google.com on the revert to ensure that a human
is aware of the problem.

To file a bug in Skia: https://bugs.chromium.org/p/skia/issues/entry
To file a bug in Flutter: https://github.com/flutter/flutter/issues/new/choose

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/main/autoroll/README.md

DEPS

@@ -18,7 +18,7 @@ vars = {
   'llvm_git': 'https://llvm.googlesource.com',
   # OCMock is for testing only so there is no google clone
   'ocmock_git': 'https://github.com/erikdoe/ocmock.git',
-  'skia_revision': '78f3a9a4587ce50769e5c2daaa480b5ec77967ce',
+  'skia_revision': '6920730bc2e8940d569f51eb2c1157aac0d0136c',
 
   # WARNING: DO NOT EDIT canvaskit_cipd_instance MANUALLY
   # See `lib/web_ui/README.md` for how to roll CanvasKit to a new version.