If we only invoke the bootstrap-enabled script when PdfJs.enabled is
true, then we don't need to check it again in the script.
This avoids a sync IPC call to the parent process.
It also keeps PdfJs.jsm from importing PdfjsContentUtils.jsm in the
child process until it is actually needed, which is one steps towards
not loading it until it is really needed.
pdfjschildbootstrap.js will always be run, but
pdfjschildbootstrap-enabled.js will only be run if PdfJs.enabled is
true. This will let us avoid some work in the child process in the
next patch.
This will need to be landed in the mozilla-central repository at the
same time as a change to nsBrowserGlue.js. See bug 1352218.