Fix problem that env(doc-title) etc. doesn't work when used as a part…

… of the `content` property value list.

`env(pub-title)` and `env(doc-title)` implemented in #512 had problem that it doesn't work when used as a part of the `content` property value list, e.g.:

```
content: env(doc-title) " (Page: " counter(page) ")" ;
```

Now, this works.

src/adapt/cssvalid.js

@@ -604,7 +604,12 @@ adapt.cssvalid.PrimitiveValidator.prototype.visitFunc = func => null;
 /**
  * @override
  */
-adapt.cssvalid.PrimitiveValidator.prototype.visitExpr = expr => expr; // null;
+adapt.cssvalid.PrimitiveValidator.prototype.visitExpr = function(expr) {
+    if (this.allowed & 0x7FE) { // ALLOW_STR|ALLOW_IDENT|...|ALLOW_ZERO_PERCENT
+        return expr;
+    }
+    return null;
+};
 
 /**
  * @param {adapt.cssvalid.PrimitiveValidator} other

src/adapt/vtree.js

@@ -1638,8 +1638,13 @@ adapt.vtree.ContentPropertyHandler.prototype.visitSpaceList = function(list) {
 /** @override */
 adapt.vtree.ContentPropertyHandler.prototype.visitExpr = function(expr) {
     const ex = expr.toExpr();
-    const val = ex.evaluate(this.context);
+    let val = ex.evaluate(this.context);
     if (typeof val === "string") {
+        if (ex instanceof adapt.expr.Named) {
+            // For env(pub-title) and env(doc-title)
+            // Need to unquote the result. To be consistent with cssparse.evaluateExprToCSS()
+            val = adapt.cssparse.parseValue(ex.scope, new adapt.csstok.Tokenizer(val, null), "").stringValue();
+        }
         goog.asserts.assert(this.elem.ownerDocument);
         const node = this.exprContentListener(ex, val, this.elem.ownerDocument);
         this.visitStrInner(val, node);