emulate srgb format for generateMipmap

gpu/command_buffer/service/gles2_cmd_srgb_converter.h

Index: gpu/command_buffer/service/gles2_cmd_srgb_converter.h
diff --git a/gpu/command_buffer/service/gles2_cmd_srgb_converter.h b/gpu/command_buffer/service/gles2_cmd_srgb_converter.h
index f711fc8daddebe45c6fd96684902063b11d76499..ddd19f6bab11783f8d5cc44c91b032c579779610 100644
--- a/gpu/command_buffer/service/gles2_cmd_srgb_converter.h
+++ b/gpu/command_buffer/service/gles2_cmd_srgb_converter.h
@@ -50,6 +50,10 @@ class GPU_EXPORT SRGBConverter {
       bool decode,
       bool encode);
 
+  void SRGBGenerateMipmap(const gles2::GLES2Decoder* decoder,
+                          Texture* tex,
+                          GLenum target);
+
  private:
   void InitializeSRGBConverterProgram();
   scoped_refptr<const gles2::FeatureInfo> feature_info_;