Make SkGpuDevice know its alpha type

src/gpu/GrSurfacePriv.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrSurfacePriv_DEFINED
 #define GrSurfacePriv_DEFINED
 
 #include "GrSurface.h"
 
 /** Class that adds methods to GrSurface that are only intended for use internal to Skia.
     This class is purely a privileged window into GrSurface. It should never have additional data
     members or virtual methods.
     Non-static methods that are not trivial inlines should be spring-boarded (e.g. declared and
     implemented privately in GrSurface with a inline public method here). */
 class GrSurfacePriv {
 public:
     /**
-     * Derive a SkImageInfo from the surface's descriptor. This is lossy as ImageInfo has fields not
-     * known to GrSurface (e.g. alphaType).
+     * Derive a SkImageInfo from the surface's descriptor. The caller must provide the alpha type as
+     * GrSurface has no equivalent.
      */
-    SkImageInfo info() const { return fSurface->info(); }
+    SkImageInfo info(SkAlphaType alphaType) const { return fSurface->info(alphaType); }
 
     /**
      * Write the contents of the surface to a PNG. Returns true if successful.
      * @param filename      Full path to desired file
      */
     bool savePixels(const char* filename) { return fSurface->savePixels(filename); }
 
     bool hasPendingRead() const { return fSurface->hasPendingRead(); }
     bool hasPendingWrite() const { return fSurface->hasPendingWrite(); }
     bool hasPendingIO() const { return fSurface->hasPendingIO(); }
@@ skipping 12 matching lines @@
     friend class GrSurface; // to construct/copy this type.
 };
 
 inline GrSurfacePriv GrSurface::surfacePriv() { return GrSurfacePriv(this); }
 
 inline const GrSurfacePriv GrSurface::surfacePriv() const {
     return GrSurfacePriv(const_cast<GrSurface*>(this));
 }
 
 #endif