More refactoring for Viewer

tools/viewer/sk_app/WindowContext.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 WindowContext_DEFINED
+#define WindowContext_DEFINED
+
+#include "GrTypes.h"
+
+class SkSurface;
+
+namespace sk_app {
+
+// TODO: fill this out with an interface
+class WindowContext {
+public:
+    virtual ~WindowContext() {}
+
+    virtual SkSurface* getBackbufferSurface() = 0;
+
+    virtual void swapBuffers() = 0;
+
+    virtual bool makeCurrent() = 0;
+
+    virtual bool isValid() = 0;
+
+    virtual void resize(uint32_t w, uint32_t h) = 0;
+
+    virtual GrBackendContext getBackendContext() = 0;
+};
+
+}   // namespace sk_app
+
+#endif