make context() virtual on device

include/core/SkDevice.h

 /*
  * Copyright 2010 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkDevice_DEFINED
 #define SkDevice_DEFINED
 
@@ skipping 262 matching lines @@
     bool readPixels(const SkImageInfo&, void* dst, size_t rowBytes, int x, int y);
 
     ///////////////////////////////////////////////////////////////////////////
 
     /** Update as needed the pixel value in the bitmap, so that the caller can
         access the pixels directly.
         @return The device contents as a bitmap
     */
     virtual const SkBitmap& onAccessBitmap() = 0;
 
+    virtual GrContext* context() const { return nullptr; }
+
 protected:
     virtual sk_sp<SkSurface> makeSurface(const SkImageInfo&, const SkSurfaceProps&);
     virtual bool onPeekPixels(SkPixmap*) { return false; }
 
     /**
      *  The caller is responsible for "pre-clipping" the dst. The impl can assume that the dst
      *  image at the specified x,y offset will fit within the device's bounds.
      *
      *  This is explicitly asserted in readPixels(), the public way to call this.
      */
@@ skipping 106 matching lines @@
     SkSurfaceProps fSurfaceProps;
 
 #ifdef SK_DEBUG
     bool        fAttachedToCanvas;
 #endif
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif