move subrun code to anonymous namespace

Change-Id: Ic4325a5529ba354c0ce176c4299a9c179f911ff2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/332136
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>

Author: herbderby

include/core/SkFont.h

@@ -510,8 +510,6 @@ class SK_API SkFont {
     SkScalar setupForAsPaths(SkPaint*);
     bool hasSomeAntiAliasing() const;
 
-    friend class GrSDFTSubRun;
-    friend class GrTextBlob;
     friend class SkFontPriv;
     friend class SkGlyphRunListPainter;
     friend class SkTextBlobCacheDiffCanvas;