Roll Skia from 25f00cb247f2 to 6a4613b83365 (3 revisions) (flutter#175087)

https://skia.googlesource.com/skia.git/+log/25f00cb247f2..6a4613b83365

2025-09-08 syoussefi@google.com [vulkan] Fix threading issues with VMA
in tests
2025-09-08 robertphillips@google.com [graphite] Add a non-threaded
PipelineManager
2025-09-08 mike@reedtribe.org Silence unuseful printfs in tests

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 aaclarke@google.com,brettos@google.com,kjlubick@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://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': '25f00cb247f23b4a8cbe7a1245bdf609fa0be846',
+  'skia_revision': '6a4613b83365adebdd342fba141916bbffaf2ca8',
 
   # WARNING: DO NOT EDIT canvaskit_cipd_instance MANUALLY
   # See `lib/web_ui/README.md` for how to roll CanvasKit to a new version.