Implement Raster Backend on Android Viewer App

tools/viewer/sk_app/RasterWindowContext.h

+
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#ifndef RasterWindowContext_DEFINED
+#define RasterWindowContext_DEFINED
+
+#include "WindowContext.h"
+
+namespace sk_app {
+
+class RasterWindowContext : public WindowContext {
+public:
+    // This is defined in the platform .cpp file
+    static RasterWindowContext* Create(void* platformData, const DisplayParams& params);
+
+    // Explicitly convert nullptr to GrBackendContext is needed for compiling
+    GrBackendContext getBackendContext() override { return (GrBackendContext) nullptr; }
+
+protected:
+    bool isGpuContext() override { return false; }
+};
+
+}   // namespace sk_app
+
+#endif