Roll Skia from 2604a89d3353 to 342273cbaa21 (6 revisions) (#19723)

https://skia.googlesource.com/skia.git/+log/2604a89d3353..342273cbaa21

2020-07-13 herb@google.com remove firstSubRun
2020-07-13 egdaniel@google.com Add GrContext api to update compressed backend textures.
2020-07-13 herb@google.com Make many things private in SubRun
2020-07-13 robertphillips@google.com Retract GrContext.h from headers
2020-07-13 herb@google.com move regenerate() from VertexRegenerator to SubRun
2020-07-13 reed@google.com remove helper now that g3 is updated

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 nifong@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

@@ -26,7 +26,7 @@ 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': '2604a89d3353ae6a6ae24081008f1e507cd2d472',
+  'skia_revision': '342273cbaa218bf3f07e3ad01c49b2c5893a5aa6',
 
   # 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

-Signature: dbe55be824c55eb414cf9c43245ac6d7
+Signature: b12325468e25e763c21f4ed9b2f6cf20
 
 UNUSED LICENSES: