Only allow calc() combinations as per spec.

We allowed calc expressions combining numbers and lengths, numbers and
percentages, and even a combination of numbers, percentages, and
lengths. None of those are allowed per the CSS Values and Units spec,
and it made it possible to combine user units, lengths, and percentages
into the same calc() for certain SVG CSS properties and attributes.

See also: w3c/csswg-drafts#4874

This improves SVG interop with Firefox. Firefox does not support calc()
for numbers, though.

Bug: 1061714
Change-Id: I9e5c492122242e51064310a40e28a233530e357c

svg/animations/svglength-additive-by-8.html

@@ -13,8 +13,8 @@
 <!-- an1: Change width from 10 to 50 in 4s -->
 <!-- an2: Change width from 10 to 100 in 4s starting at 5s -->
 <rect width="10" height="100" fill="green">
-    <animate id="an1" attributeType="XML" attributeName="width" fill="remove" by="calc(4% + 8)" begin="0s" dur="4s"/>
-        <animate id="an2" attributeType="XML" attributeName="width" additive="replace" fill="freeze" by="calc(10% + 10)" begin="5s" dur="4s"/>
+    <animate id="an1" attributeType="XML" attributeName="width" fill="remove" by="calc(4% + 8px)" begin="0s" dur="4s"/>
+        <animate id="an2" attributeType="XML" attributeName="width" additive="replace" fill="freeze" by="calc(10% + 10px)" begin="5s" dur="4s"/>
 </rect>
 
 </svg>
@@ -73,4 +73,4 @@
 
 window.animationStartsImmediately = true;
 
-</script>
+</script>