Bug 1604498: Downgrade a SVGAttrTearoffTable leak-checking assertion.…

… r=mccr8

The assertion assumes that, at shutdown time, JS will have dropped all
references to any SVG tearoff objects that we had previously handed out (where
we generated an object for JS to use to access interfaces on some SVG
attributes).

In the testcases on this bug, this assumption doesn't seem to hold (JS never
releases an object that we returned to it). But it's not particularly
concerning.  There's a small chance this represents a leak, but it's more
likely that we're just getting ahead of ourselves and quitting before we finish
freeing everything, or something to that effect.

Differential Revision: https://phabricator.services.mozilla.com/D179168

dom/svg/SVGAttrTearoffTable.h

@@ -28,7 +28,7 @@ class SVGAttrTearoffTable {
  public:
 #ifdef DEBUG
   ~SVGAttrTearoffTable() {
-    MOZ_ASSERT(!mTable, "Tear-off objects remain in hashtable at shutdown.");
+    NS_ASSERTION(!mTable, "Tear-off objects remain in hashtable at shutdown.");
   }
 #endif