Bug 1389450 - Use a valid value for -moz-force-broken-image test. r=daisuke

From the official link for -moz-force-broken-image:

 A value of 1 means that the broken image icon is even shown if the image has
 an alt attribute. A value of 0 only displays the alt attribute.

It's really ambiguous how we handle other values. Actually stylo's parser
rejects other values.  We should use the valid values for this test.

MozReview-Commit-ID: CzmIvsmauke

--HG--
extra : rebase_source : a6387e07b3578cf811bd17f772d791ec73bfa1e7

dom/animation/test/mozilla/file_discrete-animations.html

@@ -63,7 +63,7 @@ const gMozillaSpecificProperties = {
   "-moz-force-broken-image-icon": {
     // https://developer.mozilla.org/en/docs/Web/CSS/-moz-force-broken-image-icon
     from: "1",
-    to: "5"
+    to: "0"
   },
   "image-rendering": {
     // https://drafts.csswg.org/css-images-3/#propdef-image-rendering