Fix WebGLDebugShaders::getTranslatedShaderSource returning null

Source/platform/graphics/test/MockWebGraphicsContext3D.h

 /*
  * Copyright (C) 2011 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 266 matching lines @@
     virtual void texStorage2DEXT(WGC3Denum target, WGC3Dint levels, WGC3Duint internalformat, WGC3Dint width, WGC3Dint height) { }
 
     virtual WebGLId createQueryEXT() { return 1; }
     virtual void deleteQueryEXT(WebGLId) { }
     virtual GLboolean isQueryEXT(WebGLId) { return true; }
     virtual void beginQueryEXT(GLenum, WebGLId) { }
     virtual void endQueryEXT(GLenum) { }
     virtual void getQueryivEXT(GLenum, GLenum, GLint*) { }
     virtual void getQueryObjectuivEXT(WebGLId, GLenum, GLuint*) { }
 
+    virtual WebString getTranslatedShaderSourceANGLE(WebGLId) { return WebString(); }
+
 protected:
     unsigned m_nextTextureId;
     Attributes m_attrs;
 };
 
 } // namespace blink
 
 #endif // MockWebGraphicsContext3D_h