Add factory class for generating various flavors of GrTextContext.

include/gpu/GrDistanceFieldTextContext.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrDistanceFieldTextContext_DEFINED
 #define GrDistanceFieldTextContext_DEFINED
 
 #include "GrTextContext.h"
 
 class GrTextStrike;
 
 /*
  * This class implements GrTextContext using distance field fonts
  */
 class GrDistanceFieldTextContext : public GrTextContext {
 public:
-    GrDistanceFieldTextContext(GrContext*, const GrPaint&, const SkPaint&);
-    virtual ~GrDistanceFieldTextContext();
-
     virtual void drawPackedGlyph(GrGlyph::PackedID, GrFixed left, GrFixed top,
                                  GrFontScaler*) SK_OVERRIDE;
 
     void drawText(const char text[], size_t byteLength,
                   SkScalar x, SkScalar y, SkGlyphCache*, GrFontScaler*);
     void drawPosText(const char text[], size_t byteLength,
                      const SkScalar pos[], SkScalar constY,
                      int scalarsPerPosition,
                      SkGlyphCache* cache, GrFontScaler* fontScaler);
 
     const SkPaint& getSkPaint() { return fSkPaint; }
 
 private:
+    GrDistanceFieldTextContext(GrContext*, const GrPaint&, const SkPaint&);
+    virtual ~GrDistanceFieldTextContext();
+    friend class GrTTextContextManager<GrDistanceFieldTextContext>;
+
     GrTextStrike*           fStrike;
     SkScalar                fTextRatio;
 
     void flushGlyphs();                 // automatically called by destructor
 
     enum {
         kMinRequestedGlyphs      = 1,
         kDefaultRequestedGlyphs  = 64,
         kMinRequestedVerts       = kMinRequestedGlyphs * 4,
         kDefaultRequestedVerts   = kDefaultRequestedGlyphs * 4,
     };
 
-    SkPaint                 fSkPaint;
     SkPoint*                fVertices;
     int32_t                 fMaxVertices;
     GrTexture*              fCurrTexture;
     int                     fCurrVertex;
 };
 
 #endif