Move compute_combined_buffer_size behind GR_TEST_UTILS

Change-Id: I7df21ef7f3b491be5d54477f2429e4ab7603e14b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213671
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed