A JavaScript function to inject. This function will be serialized, and
then deserialized for injection. This means that any bound parameters
and execution context will be lost.
Exactly one of files or func must be
specified.
We used to call the injected function function, but this is
incompatible with JavaScript's object declaration shorthand (see
https://crbug.com/1166438). We leave this silently in for backwards
compatibility.
TODO(devlin): Remove this in M95.
Whether the injection should be triggered in the target as soon as
possible. Note that this is not a guarantee that injection will occur
prior to page load, as the page may have already loaded by the time the
script reaches the target.