Move canvas->surface association to the device subclasses

src/gpu/SkGpuDevice.h

Index: src/gpu/SkGpuDevice.h
diff --git a/src/gpu/SkGpuDevice.h b/src/gpu/SkGpuDevice.h
index 9eddc41a21521153902ac8b27037b4fc56f2c37d..b2419601fa908c22cf00498498a6f68b3222c57b 100644
--- a/src/gpu/SkGpuDevice.h
+++ b/src/gpu/SkGpuDevice.h
@@ -24,6 +24,7 @@ struct GrSkDrawProcs;
 class GrAccelData;
 struct GrCachedLayer;
 class GrTextContext;
+class SkSurface_Gpu;
 
 /**
  *  Subclass of SkBaseDevice, which directs all drawing to the GrGpu owned by the
@@ -50,6 +51,10 @@ public:
 
     virtual ~SkGpuDevice();
 
+    void setSurface(SkSurface_Gpu* surface) {
+        fSurface = surface;
+    }
+
     /** Creates a render targetthat can be used as the render target of the device. */
     static GrRenderTarget* CreateRenderTarget(GrContext* context, SkSurface::Budgeted budgeted,
                                               const SkImageInfo& info, int sampleCount);
@@ -66,7 +71,7 @@ public:
     // set all pixels to 0
     void clearAll();
 
-    void swapRenderTarget(GrRenderTarget* newTarget);
+    void detachBackendRenderTarget(bool retainContent);
 
     GrRenderTarget* accessRenderTarget() SK_OVERRIDE;
 
@@ -76,6 +81,8 @@ public:
 
     const SkSurfaceProps& surfaceProps() const { return fSurfaceProps; }
 
+    void discard() SK_OVERRIDE;
+
     void drawPaint(const SkDraw&, const SkPaint& paint) SK_OVERRIDE;
     virtual void drawPoints(const SkDraw&, SkCanvas::PointMode mode, size_t count,
                             const SkPoint[], const SkPaint& paint) SK_OVERRIDE;
@@ -149,6 +156,7 @@ private:
     // remove when our clients don't rely on accessBitmap()
     SkBitmap        fLegacyBitmap;
     bool            fNeedClear;
+    SkSurface_Gpu*  fSurface;
 
     SkGpuDevice(GrRenderTarget*, const SkSurfaceProps*, unsigned flags);