While working through a change that adds a Blink-variant of a Mojom definition, I hit a case where the browser and renderer processes were sending data via a string type that contained invalid UTF-8.  Previously, both sides of the pipe were in the Content layer and both used std::string, so sending data this way, while bad practice, was technically possible and worked fine.

The new Blink-variant I added auto-maps that string type to WTF::String via Mojo's C++ traits.  string_traits_wtf.* uses String::FromUTF8() which invalidates string data that contains invalid UTF-8.  As a result, the Blink-variant string was getting set to the null string and later codepaths that assumed correct data was received failed.  We plan to fix the design issue in our Mojom definition by changing that type from string to array<uint8>.

As an idea for improvement, if Mojo alerted when string types contain invalid UTF-8, the early notification would have helped us find and fix a design issue before the potential for data loss.  Filing here after discussing with Ken.