Add factory class for generating various flavors of GrTextContext.

include/gpu/GrDistanceFieldTextContext.h

Index: include/gpu/GrDistanceFieldTextContext.h
diff --git a/include/gpu/GrDistanceFieldTextContext.h b/include/gpu/GrDistanceFieldTextContext.h
index 3e00ff2477c4f91be0a793eeab540289b28776cc..4fde6ceef8d8b8dc3e8375c9732d4f14d7211466 100755
--- a/include/gpu/GrDistanceFieldTextContext.h
+++ b/include/gpu/GrDistanceFieldTextContext.h
@@ -17,9 +17,6 @@ class GrTextStrike;
  */
 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;
 
@@ -33,6 +30,10 @@ public:
     const SkPaint& getSkPaint() { return fSkPaint; }
 
 private:
+    GrDistanceFieldTextContext(GrContext*, const GrPaint&, const SkPaint&);
+    virtual ~GrDistanceFieldTextContext();
+    friend class GrTTextContextManager<GrDistanceFieldTextContext>;
+
     GrTextStrike*           fStrike;
     SkScalar                fTextRatio;
 
@@ -45,7 +46,6 @@ private:
         kDefaultRequestedVerts   = kDefaultRequestedGlyphs * 4,
     };
 
-    SkPaint                 fSkPaint;
     SkPoint*                fVertices;
     int32_t                 fMaxVertices;
     GrTexture*              fCurrTexture;