Add support for EXT_image_flush_external extension

gpu/command_buffer/tests/texture_image_factory.cc

Index: gpu/command_buffer/tests/texture_image_factory.cc
diff --git a/gpu/command_buffer/tests/texture_image_factory.cc b/gpu/command_buffer/tests/texture_image_factory.cc
index d34e5e3e8d8c199c019547f32b0c1caac15cc245..6144e0ff7e03cf61af465a532ef37b3eaea5be4d 100644
--- a/gpu/command_buffer/tests/texture_image_factory.cc
+++ b/gpu/command_buffer/tests/texture_image_factory.cc
@@ -41,6 +41,8 @@ class TextureImage : public gl::GLImage {
     return false;
   }
 
+  void Flush() override {}
+
   void OnMemoryDump(base::trace_event::ProcessMemoryDump* pmd,
                     uint64_t process_tracing_id,
                     const std::string& dump_name) override {}