Actually make GrContextOptions::fUseNGA GR_TEST_UTILS-only

I accidentally left this out of:

https://skia-review.googlesource.com/c/skia/+/415796 (Add option to switch between OGA and NGA SkGpuDevices)

Bug: skia:11837
Change-Id: I49730b1f338f081512a7fca2d0ca6e1d4cf086be
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/417536
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>

include/gpu/GrContextOptions.h

@@ -267,12 +267,6 @@ struct SK_API GrContextOptions {
      */
     bool fEnableExperimentalHardwareTessellation = false;
 
-    /**
-     * Experimental: Should the Ganesh' Next Generation Architecture be used instead of the
-     * old architecture?
-     */
-    Enable fUseNGA = Enable::kDefault;
-
     /**
      * Uses a reduced variety of shaders. May perform less optimally in steady state but can reduce
      * jank due to shader compilations.
@@ -284,6 +278,12 @@ struct SK_API GrContextOptions {
      * Private options that are only meant for testing within Skia's tools.
      */
 
+    /**
+     * Experimental: Should the Ganesh' Next Generation Architecture be used instead of the
+     * old architecture?
+     */
+    Enable fUseNGA = Enable::kDefault;
+
     /**
      * Prevents use of dual source blending, to test that all xfer modes work correctly without it.
      */