Roll Skia from 0ee149894a8d to 21c6fb26c14e (1 revision) (#52991)

https://skia.googlesource.com/skia.git/+log/0ee149894a8d..21c6fb26c14e

2024-05-22 michaelludwig@google.com [graphite] Use scratch textures for async readback

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,jlavrova@google.com,jonahwilliams@google.com,rmistry@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

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': '0ee149894a8d24e7b409502d3efa879e6cb2e3c7',
+  'skia_revision': '21c6fb26c14e30f309219a4a5eb66fc1b820988e',
 
   # WARNING: DO NOT EDIT canvaskit_cipd_instance MANUALLY
   # See `lib/web_ui/README.md` for how to roll CanvasKit to a new version.

ci/licenses_golden/licenses_skia

@@ -1,4 +1,4 @@
-Signature: a7f87aa6ac93b5e8673851360ff917c0
+Signature: b31276c9f255bce1607949b4d17a884b
 
 ====================================================================================================
 LIBRARY: etc1