Roll Skia from 4b788d0e5e63 to 721e68fe652a (2 revisions) (flutter#174095)

https://skia.googlesource.com/skia.git/+log/4b788d0e5e63..721e68fe652a

2025-08-20 [email protected] Roll Dawn
from bfb0f1fba4a0 to 555275f0c242 (9 revisions)
2025-08-20 [email protected] Roll
vulkan-deps from 6ab6cf8994f3 to 607fb6b15ab2 (6 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 [email protected],[email protected],[email protected] 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://issues.skia.org/issues/new?component=1389291&template=1850622

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

Author: engine-flutter-autoroll

DEPS

@@ -14,7 +14,7 @@ vars = {
   'flutter_git': 'https://flutter.googlesource.com',
   'skia_git': 'https://skia.googlesource.com',
   'llvm_git': 'https://llvm.googlesource.com',
-  'skia_revision': '4b788d0e5e639aa81b4ff71e7dcb15710d720593',
+  'skia_revision': '721e68fe652a77cbc283dce2b825f2437ecc107a',
 
   # WARNING: DO NOT EDIT canvaskit_cipd_instance MANUALLY
   # See `lib/web_ui/README.md` for how to roll CanvasKit to a new version.