Implement OpenGL backend in Android viewer app

tools/viewer/sk_app/GLWindowContext.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 GLWindowContext_DEFINED
 #define GLWindowContext_DEFINED
 
@@ skipping 34 matching lines @@
     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
+    // Note that the platform .cpp file is responsible for
+    // initializing fSampleCount, fStencilBits, and fColorBits!
     int                               fSampleCount;
     int                               fStencilBits;
     int                               fColorBits;
     int                               fActualColorBits;
 };
 
 }   // namespace sk_app
 
 #endif