Hide SkTextBlob::GlyphPositioning from public API

Make private, add corresponding SkTextBlobRunIterator enum.

Also move SkTextBlobRunIterator to SkTextBlobPriv.h.

Change-Id: If3e0beabb0e8484b1ac73c6fc0827e920ca1f93b
Reviewed-on: https://skia-review.googlesource.com/142501
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
diff --git a/include/core/SkTextBlob.h b/include/core/SkTextBlob.h
index cf77a78..8ed66da 100644
--- a/include/core/SkTextBlob.h
+++ b/include/core/SkTextBlob.h
@@ -36,12 +36,6 @@
     static sk_sp<SkTextBlob> MakeAsDrawText(
             const void* text, size_t byteLength, const SkPaint& paint);
 
-    enum GlyphPositioning : uint8_t {
-        kDefault_Positioning      = 0, // Default glyph advances -- zero scalars per glyph.
-        kHorizontal_Positioning   = 1, // Horizontal positioning -- one scalar per glyph.
-        kFull_Positioning         = 2  // Point positioning -- two scalars per glyph.
-    };
-
     /**
      *  Similar to serialize above, but writes directly into |memory|. Returns bytes written or 0u
      *  if serialization failed due to insufficient size.
@@ -57,6 +51,8 @@
     friend class SkNVRefCnt<SkTextBlob>;
     class RunRecord;
 
+    enum GlyphPositioning : uint8_t;
+
     explicit SkTextBlob(const SkRect& bounds);
 
     ~SkTextBlob();