Make current GPU blurring implementation be OGA-only

Bug: skia:11837
Change-Id: I8b209b362238e18a110fc518cb1c0b2f2244c22f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/419840
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
8 files changed