Add OpenGL context to Viewer.

tools/viewer/sk_app/WindowContext.h

Index: tools/viewer/sk_app/WindowContext.h
diff --git a/tools/viewer/sk_app/WindowContext.h b/tools/viewer/sk_app/WindowContext.h
index d48e38e7466949b07031a3dd2443231f3011e847..4ed1f119ea403749e1bed716aa24602d5a0d515e 100644
--- a/tools/viewer/sk_app/WindowContext.h
+++ b/tools/viewer/sk_app/WindowContext.h
@@ -9,22 +9,25 @@
 
 #include "DisplayParams.h"
 #include "GrTypes.h"
+#include "SkRefCnt.h"
+#include "SkSurfaceProps.h"
 
+class GrContext;
 class SkSurface;
 
 namespace sk_app {
 
-// TODO: fill this out with an interface
 class WindowContext {
 public:
+    WindowContext() : fContext(nullptr)
+                    , fSurfaceProps(SkSurfaceProps::kLegacyFontHost_InitType) {}
+
     virtual ~WindowContext() {}
 
-    virtual SkSurface* getBackbufferSurface() = 0;
+    virtual sk_sp<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;
@@ -33,6 +36,13 @@ public:
     virtual void setDisplayParams(const DisplayParams& params) = 0;
 
     virtual GrBackendContext getBackendContext() = 0;
+
+protected:
+    GrContext*        fContext;
+
+    DisplayParams     fDisplayParams;
+    GrPixelConfig     fPixelConfig;
+    SkSurfaceProps    fSurfaceProps;
 };
 
 }   // namespace sk_app