Add support for EXT_image_flush_external extension

gpu/command_buffer/service/gles2_cmd_decoder_unittest_textures.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder_unittest_textures.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder_unittest_textures.cc b/gpu/command_buffer/service/gles2_cmd_decoder_unittest_textures.cc
index bc949c63cace3d597bd0eba82892412e3d29a7ee..38c8ecd8333ac820aa81f826c16857e3462c5fc8 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder_unittest_textures.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder_unittest_textures.cc
@@ -3029,6 +3029,7 @@ class MockGLImage : public gl::GLImage {
                void(base::trace_event::ProcessMemoryDump*,
                     uint64_t,
                     const std::string&));
+  void Flush() override {}
 
  protected:
   virtual ~MockGLImage() {}