Issue #1885 - Allow unitless rootMargin entries for IntersectionObserver.

I could have done this through a CSSLoader to allow all CSS unit quirks but
I wasn't planning to start passing around document and element references
everywhere, so instead just did it manually by accepting numbers/floats in
addition to pixel and percent.

layout/style/nsCSSParser.cpp

@@ -2315,7 +2315,7 @@ CSSParserImpl::ParseMarginString(const nsSubstring& aBuffer,
   } else {
     UngetToken();
     // Parse a margin, and check that there's nothing else after it.
-    marginParsed = ParseGroupedBoxProperty(VARIANT_LP, aValue, 0) &&
+    marginParsed = ParseGroupedBoxProperty(VARIANT_LPN, aValue, 0) &&
                    !GetToken(true);
   }