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/src/gpu/text/GrTextBlobCache.h b/src/gpu/text/GrTextBlobCache.h
index 110e0ad..fe245c9 100644
--- a/src/gpu/text/GrTextBlobCache.h
+++ b/src/gpu/text/GrTextBlobCache.h
@@ -12,7 +12,7 @@
 #include "SkMessageBus.h"
 #include "SkRefCnt.h"
 #include "SkTArray.h"
-#include "SkTextBlobRunIterator.h"
+#include "SkTextBlobPriv.h"
 #include "SkTHash.h"
 
 class GrTextBlobCache {