Make GrGLGpu's program cache be held as a unique_ptr

This seems a little bit less fussy.

Change-Id: If62fca188c83f1317351e1f18342a26e54355c07
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/271597
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed