Add flag for SkGpuSurface creation to enable distance fields.

src/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 SkDeviceProperties&);
+    GrDistanceFieldTextContext(GrContext*, const SkDeviceProperties&, bool enable);
     virtual ~GrDistanceFieldTextContext();
 
     virtual void drawText(const GrPaint&, const SkPaint&, const char text[], size_t byteLength,
                           SkScalar x, SkScalar y) SK_OVERRIDE;
     virtual void drawPosText(const GrPaint&, const SkPaint&,
                              const char text[], size_t byteLength,
                              const SkScalar pos[], SkScalar constY,
                              int scalarsPerPosition) SK_OVERRIDE;
 
     virtual bool canDraw(const SkPaint& paint) SK_OVERRIDE;
 
 private:
     GrTextStrike*           fStrike;
     SkScalar                fTextRatio;
     bool                    fUseLCDText;
+    bool                    fEnableDFRendering;
 
     void init(const GrPaint&, const SkPaint&);
     void drawPackedGlyph(GrGlyph::PackedID, SkFixed left, SkFixed top, GrFontScaler*);
     void flushGlyphs();                 // automatically called by destructor
     void finish();
 
     enum {
         kMinRequestedGlyphs      = 1,
         kDefaultRequestedGlyphs  = 64,
         kMinRequestedVerts       = kMinRequestedGlyphs * 4,
         kDefaultRequestedVerts   = kDefaultRequestedGlyphs * 4,
     };
 
     SkPoint*                fVertices;
     int32_t                 fMaxVertices;
     GrTexture*              fCurrTexture;
     int                     fCurrVertex;
 };
 
 #endif