If no header includes the prototype of a function, then it cannot be
used from outside the translation unit it was defined in. In that case,
it should be marked as `static`, in order to avoid possible ODR
problems, unnecessary exported symbols, and allow the compiler to better
optimize those.
If this warning triggers in a function defined in a header, `inline`
needs to be added, otherwise if the header is included in more than one
TU, it will fail to link with a duplicate definition error.
The reason this diff got so big is that Lagom-only code wasn't built
with this flag even in Serenity times.
When launched with the new --enable-idl-tracing option, we now log
every call to web platform APIs declared via IDL, along with the
arguments passed.
This can be very helpful when trying to figure out what a site is
doing, especially if it's not doing what you'd expect.
