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 17a21957f7dd58a38fb1aa67bb2436275199315d..2a19fa9fd8e104a8c6256f533da54a3329f5c646 100644
--- a/gpu/command_buffer/service/gles2_cmd_srgb_converter.h
+++ b/gpu/command_buffer/service/gles2_cmd_srgb_converter.h
@@ -51,6 +51,10 @@ class GPU_EXPORT SRGBConverter {
       bool encode,
       bool enable_scissor_test);
 
+  void GenerateMipmap(const gles2::GLES2Decoder* decoder,
+                      Texture* tex,
+                      GLenum target);
+
  private:
   void InitializeSRGBConverterProgram();
   scoped_refptr<const gles2::FeatureInfo> feature_info_;