Fix undefined maxFontSize

Fix getting a NaN maxFontSize in makeVList because symbolNodes don't have a
maxFontSize property.

Test plan:
 - Make sure no huxley screenshots changed

Auditors: alpert

domTree.js

@@ -65,6 +65,7 @@ function symbolNode(value, height, depth, italic, classes, style) {
     this.italic = italic || 0;
     this.classes = classes || [];
     this.style = style || {};
+    this.maxFontSize = 0;
 }
 
 symbolNode.prototype.toDOM = function() {