Bug 1510985 - Remove Event.returnValue temporarily in 64. r=smaug a=jcristau

Event.returnValue causes breaking a bank application. The reason of the
breakage is, the web app assumes that Event.returnValue is available only
on IE and it hits same incompatibility issue with window.event (bug 1479964).
Additionally, the app may be used in various domains including in intranet.
Therefore, we cannot disable it with blacklist of domains like other similar
changes.

Fortunately, we can ship new keypress event behaviors which is
tracked by bug 1479964 in 65. So, for backward compatibility with 62 or
earlier, we should temporarily disable Event.returnValue in 64.

But unfortunately, this was introduced in 63. So, if some web apps have
started to use this legacy attribute, we need to contact their vendors, though.