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/HTMLImageElement.cpp

@@ -528,11 +528,8 @@ bool HTMLImageElement::IsHTMLFocusable(bool aWithMouse, bool* aIsFocusable,
     *aTabIndex = (sTabFocusModel & eTabFocus_formElementsMask) ? tabIndex : -1;
   }
 
-  *aIsFocusable =
-#ifdef XP_MACOSX
-      (!aWithMouse || nsFocusManager::sMouseFocusesFormControl) &&
-#endif
-      (tabIndex >= 0 || GetTabIndexAttrValue().isSome());
+  *aIsFocusable = IsFormControlDefaultFocusable(aWithMouse) &&
+                  (tabIndex >= 0 || GetTabIndexAttrValue().isSome());
 
   return false;
 }