Add flag for SkGpuSurface creation to enable distance fields.

src/gpu/GrDistanceFieldTextContext.h

Index: src/gpu/GrDistanceFieldTextContext.h
diff --git a/src/gpu/GrDistanceFieldTextContext.h b/src/gpu/GrDistanceFieldTextContext.h
index 58c0824e77061d03d3bb3e5275dca2455185a0e2..3dfffd1c137addaf55506705d53bee892e4e9cf4 100644
--- a/src/gpu/GrDistanceFieldTextContext.h
+++ b/src/gpu/GrDistanceFieldTextContext.h
@@ -17,7 +17,7 @@ class GrTextStrike;
  */
 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,
@@ -33,6 +33,7 @@ private:
     GrTextStrike*           fStrike;
     SkScalar                fTextRatio;
     bool                    fUseLCDText;
+    bool                    fEnableDFRendering;
 
     void init(const GrPaint&, const SkPaint&);
     void drawPackedGlyph(GrGlyph::PackedID, SkFixed left, SkFixed top, GrFontScaler*);