disable legacy shaders in flutter

All of Flutter's non-test drawing is done with Ganesh,
and the few shaders that are legacy-only are not used by Flutter.

This should cut a bunch of essentially-dead code from Flutter, though
some of the Flutter test images may change because those test images are
software rasterized.  Hopefully nothing should look terribly different.

Looks like this cuts ~27K from Flutter.

Change-Id: I4c8078db17fe9d24b15ed9d8c6adb528a382f02a
Reviewed-on: https://skia-review.googlesource.com/c/163165
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/gn/flutter_defines.gni b/gn/flutter_defines.gni
index 88bdb44..d7ef0e3 100644
--- a/gn/flutter_defines.gni
+++ b/gn/flutter_defines.gni
@@ -12,5 +12,6 @@
   "SK_DISABLE_DAA",
 
   # Fast low-precision software rendering isn't a priority for Flutter.
+  "SK_DISABLE_LEGACY_SHADERCONTEXT",
   "SK_DISABLE_LOWP_RASTER_PIPELINE",
 ]