The loading state is only used in gr-diff, so it makes sense to be also
set in gr-diff.
This change slightly changes the behavior of loading to flip to false
when the content is rendered, not only after syntax highlighting is
done. That is not a problem though,
because the two places that read it only care about content, too.
Change-Id: I2b4bda1ff8f336bce4a7a763d1479b2474a2beb7