Add sRGB mode toggle to 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 "DisplayParams.h"
 #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 const DisplayParams& getDisplayParams() = 0;
+    virtual void setDisplayParams(const DisplayParams& params) = 0;
+
     virtual GrBackendContext getBackendContext() = 0;
 };
 
 }   // namespace sk_app
 
 #endif