Pass a non-nullable string to NamedGetter.

There is no actual reason to use a nullable string here; all callers have a
string they want to pass.

The issue dates back to the time that DOMString was inherently nullable
(before #1215); this API was not converted back to the non-nullable DOMString
type after that landed.
Remove an unused argument to needCx and use it in CGCallGenerator.
Introduce a PerformanceTimingHelpers trait for the NavigationStartPrecise method.
Remove references to nullable any from testbinding.

'any' is inherently nullable, so nullable any doesn't exist.
Comment out references to floating-point arguments with default values.

Our parser does not support floating-point constants, so we can't use default
values for floating-point types.
Add methods that return floating-point types to TestBinding.webidl.

They were already implemented in testbinding.rs, but were forgotten in the IDL
file.
Comment out the stringifier in URLSearchParams.webidl.

We don't currently support stringifiers.
Remove OverrideMimeType from XMLHttpRequestMethods.

It is not present in the IDL, and is not actually implemented.
Remove the unused lifetime variable from XMLHttpRequestMethods.

The lifetime applies to the JSRef type the trait is implemented on, not to the
trait itself.
Start work on generating traits.
Move over to the new trait.