make context() virtual on device

src/gpu/SkGpuDevice.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 SkGpuDevice_DEFINED
 #define SkGpuDevice_DEFINED
 
@@ skipping 45 matching lines @@
 
     ~SkGpuDevice() override {}
 
     SkGpuDevice* cloneDevice(const SkSurfaceProps& props) {
         SkBaseDevice* dev = this->onCreateDevice(CreateInfo(this->imageInfo(), kPossible_TileUsage,
                                                             props.pixelGeometry()),
                                                  nullptr);
         return static_cast<SkGpuDevice*>(dev);
     }
 
-    GrContext* context() const { return fContext; }
+    GrContext* context() const override { return fContext; }
 
     // set all pixels to 0
     void clearAll();
 
     void replaceRenderTarget(bool shouldRetainContent);
 
     GrRenderTarget* accessRenderTarget() override;
 
     SkImageInfo imageInfo() const override {
         return fLegacyBitmap.info();
@@ skipping 174 matching lines @@
 
     void drawSpriteWithFilter(const SkDraw&, const SkBitmap&, int x, int y,
                               const SkPaint&) override;
 
     friend class GrAtlasTextContext;
     friend class SkSurface_Gpu;      // for access to surfaceProps
     typedef SkBaseDevice INHERITED;
 };
 
 #endif