Update DDL test harness to use GrDirectContexts

Before I start updating this to support relocatable DDLs I felt I
should clean it up a bit.

Change-Id: I640d15a40164b33c4c2d7378e37d39fe7d3ff313
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/300926
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed