Require a direct context to precompile shaders in DM

Change-Id: I44051f0e00ac47c53631084e2221594ec30bddfc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/310067
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed