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/GrTextContext.cpp b/src/gpu/text/GrTextContext.cpp
index aeb99f1..d57055a 100644
--- a/src/gpu/text/GrTextContext.cpp
+++ b/src/gpu/text/GrTextContext.cpp
@@ -196,7 +196,7 @@
         }
         cacheBlob->setRunPaintFlags(run, runPaint.skPaint().getFlags());
 
-        SkASSERT(it.positioning() == SkTextBlob::kFull_Positioning);
+        SkASSERT(it.positioning() == SkTextBlobRunIterator::kFull_Positioning);
         if (CanDrawAsDistanceFields(runPaint, viewMatrix, props,
                                     shaderCaps.supportsDistanceFieldText(), fOptions)) {
             this->drawDFPosText(cacheBlob, run, glyphCache, props, runPaint,