Bug 1699570 - Make mouse_focuses_formcontrol on chrome documents match the old behavior. r=mstange

For that, we make accessibility.mouse_focuses_formcontrol a static pref,
and make it work in all platforms because it's simpler and allows to
test mac-specific things on other platforms more easily.

Differential Revision: https://phabricator.services.mozilla.com/D109006

dom/html/HTMLInputElement.cpp

@@ -6131,13 +6131,7 @@ bool HTMLInputElement::IsHTMLFocusable(bool aWithMouse, bool* aIsFocusable,
     return false;
   }
 
-#ifdef XP_MACOSX
-  const bool defaultFocusable =
-      !aWithMouse || nsFocusManager::sMouseFocusesFormControl;
-#else
-  const bool defaultFocusable = true;
-#endif
-
+  const bool defaultFocusable = IsFormControlDefaultFocusable(aWithMouse);
   if (CreatesDateTimeWidget()) {
     if (aTabIndex) {
       // We only want our native anonymous child to be tabable to, not ourself.