Add OpenGL context to Viewer.

tools/viewer/sk_app/GLWindowContext.h

Index: tools/viewer/sk_app/GLWindowContext.h
diff --git a/tools/viewer/sk_app/GLWindowContext.h b/tools/viewer/sk_app/GLWindowContext.h
new file mode 100644
index 0000000000000000000000000000000000000000..6fb5e0988924bd28d37095f793cc7328e9166cc9
--- /dev/null
+++ b/tools/viewer/sk_app/GLWindowContext.h
@@ -0,0 +1,63 @@
+
+/*
+ * 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 GLWindowContext_DEFINED
+#define GLWindowContext_DEFINED
+
+
+#include "gl/GrGLInterface.h"
+
+#include "SkRefCnt.h"
+#include "GrRenderTarget.h"
+#include "SkSurface.h"
+
+#include "WindowContext.h"
+
+class GrContext;
+
+namespace sk_app {
+
+class GLWindowContext : public WindowContext {
+public:
+    // This is defined in the platform .cpp file
+    static GLWindowContext* Create(void* platformData, const DisplayParams& params);
+
+    sk_sp<SkSurface> getBackbufferSurface() override;
+
+    bool isValid() override { return SkToBool(fBackendContext.get()); }
+
+    void resize(uint32_t w, uint32_t h) override;
+    void swapBuffers() override;
+
+    void setDisplayParams(const DisplayParams& params) override;
+
+    GrBackendContext getBackendContext() override {
+        return (GrBackendContext) fBackendContext.get();
+    }
+
+protected:
+    GLWindowContext(void*, const DisplayParams&);
+    void initializeContext(void*, const DisplayParams&);
+    virtual void onInitializeContext(void*, const DisplayParams&) = 0;
+    void destroyContext();
+    virtual void onDestroyContext() = 0;
+    virtual void onSwapBuffers() = 0;
+
+    SkAutoTUnref<const GrGLInterface> fBackendContext;
+    sk_sp<GrRenderTarget>             fRenderTarget;
+    sk_sp<SkSurface>                  fSurface;
+
+    // parameters obtained from the native window
+    int                               fSampleCount;
+    int                               fStencilBits;
+    int                               fColorBits;
+    int                               fActualColorBits;
+};
+
+}   // namespace sk_app
+
+#endif