Add API for GrContext to recommend rendertarget sample count

include/gpu/GrContext.h

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrContext_DEFINED
 #define GrContext_DEFINED
 
@@ skipping 288 matching lines @@
      * current GPU.
      */
     int getMaxRenderTargetSize() const;
 
     /**
      * Returns the max sample count for a render target. It will be 0 if MSAA
      * is not supported.
      */
     int getMaxSampleCount() const;
 
+    /**
+     * Returns the recommended sample count for a render target when using this
+     * context.
+     *
+     * @param  config the configuration of the render target.
+     * @param  dpi the display density in dots per inch.
+     *
+     * @return sample count that should be perform well and have good enough
+     *         rendering quality for the display. Alternatively returns 0 if
+     *         MSAA is not supported or recommended to be used by default.
+     */
+    int getRecommendedSampleCount(GrPixelConfig config, SkScalar dpi) const;
+
     ///////////////////////////////////////////////////////////////////////////
     // Backend Surfaces
 
     /**
      * Wraps an existing texture with a GrTexture object.
      *
      * OpenGL: if the object is a texture Gr may change its GL texture params
      *         when it is drawn.
      *
      * @param  desc     description of the object to create.
@@ skipping 725 matching lines @@
     }
 
     GrTexture* texture() { return fTexture; }
 
 private:
     GrContext*                    fContext;
     GrTexture*                    fTexture;
 };
 
 #endif