Add GL_CHROMIUM_screen_space_antialiasing to support alternative AA

public/platform/WebGraphicsContext3D.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 358 matching lines @@
     virtual void setErrorMessageCallback(WebGraphicsErrorMessageCallback* callback) { }
     // GL_ARB_robustness
     //
     // This entry point must provide slightly different semantics than
     // the GL_ARB_robustness extension; specifically, the lost context
     // state is sticky, rather than reported only once.
     virtual WGC3Denum getGraphicsResetStatusARB() { return 0; /* GL_NO_ERROR */ }
 
     virtual WebString getTranslatedShaderSourceANGLE(WebGLId shader) = 0;
 
+    // GL_CHROMIUM_screen_space_antialiasing
+    virtual void applyScreenSpaceAntialiasingCHROMIUM() { }
+
     // GL_CHROMIUM_iosurface
     virtual void texImageIOSurface2DCHROMIUM(WGC3Denum target, WGC3Dint width, WGC3Dint height, WGC3Duint ioSurfaceId, WGC3Duint plane) { }
 
     // GL_EXT_texture_storage
     virtual void texStorage2DEXT(WGC3Denum target, WGC3Dint levels, WGC3Duint internalformat,
                                  WGC3Dint width, WGC3Dint height) { }
 
     // GL_EXT_occlusion_query
     virtual WebGLId createQueryEXT() { return 0; }
     virtual void deleteQueryEXT(WebGLId query) { }
@@ skipping 171 matching lines @@
     virtual void vertexAttribI4iv(WGC3Duint index, const WGC3Dint *v) { }
     virtual void vertexAttribI4ui(WGC3Duint index, WGC3Duint x, WGC3Duint y, WGC3Duint z, WGC3Duint w) { }
     virtual void vertexAttribI4uiv(WGC3Duint index, const WGC3Duint *v) { }
     virtual void vertexAttribIPointer(WGC3Duint index, WGC3Dint size, WGC3Denum type, WGC3Dsizei stride, WGC3Dintptr pointer) { }
     virtual void waitSync(WGC3Dsync sync, WGC3Dbitfield flags, WGC3Duint64 timeout) { }
 };
 
 } // namespace blink
 
 #endif