[graphite] Always initialize test-only Context backpointer
[graphite] Always initialize test-only Context backpointer

Otherwise it's possible to segfault trying to use the uninitialized
Context pointer in Recorder's destructor.

Change-Id: Ic322a7d433a2ad55c435cd06da3be98f135fc90a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/603408
Reviewed-by: Greg Daniel <39df0a804564ccb6cf75f18db79653821f37c1c5@google.com>
Commit-Queue: James Godfrey-Kittle <9596333547d2a8c18f44ec1344e5fcade53c31bb@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia