Bug 1741469 part 3: Add an off-by-default pref to control whether input type="number"/"range" fields are modified when mousewheel-scrolled.  a=dmeehan

The C++ code change here is simply replacing an ifdef with a pref-check (and
associated reindentation for being placed in an "if"-block).

The pref is off-by-default for now, since no other browser seems to modify
these fields through mousewheel scrolling, and users seem to be surprised when
they encounter the behavior in Firefox.  See discussion on the bugzilla page.

Note: now that this behavior is controlled by a pref, we don't need to have
platform-specific #ifdefs and "skip-if" test exemptions. If we decide to bring
this behavior back on certain platforms, we can do so by simply changing the
default value of the pref on those platforms.

Original Revision: https://phabricator.services.mozilla.com/D175719

Differential Revision: https://phabricator.services.mozilla.com/D217523