Add parseFontFaceDescriptor for @font-face handling

LayoutTests/fast/css/font-face-src-parsing.html

 <!-- Valid src descriptor rules. -->
 <style>
 @font-face {
     src: url(font.ttf);
 }
 @font-face {
     src: url(http://foo/bar/font.ttf);
 }
 @font-face {
     src: url(font.svg#id);
@@ skipping 50 matching lines @@
 
 @font-face {
     src: url(foo.ttf), invalid;
 }
 @font-face {
     src: url(foo.ttf) "invalid";
 }
 @font-face {
     src: url(foo.ttf) !important;
 }
+@font-face {
+    src: initial;
+}
+@font-face {
+    src: inherit;
+}
+@font-face {
+    src: unset;
+}
 </style>
 <p>Test parsing the src descriptor of @font-face.</p>
 
 <p>Valid rules form the stylesheet:</p>
 <div id="validResult"></div>
 
 <p>Expected result for valid rules:</p>
 <pre id="validExpected">@font-face { src: url(font.ttf); }
 @font-face { src: url(http://foo/bar/font.ttf); }
 @font-face { src: url(font.svg#id); }
@@ skipping 36 matching lines @@
 document.getElementById('validResult').innerText = result;
 
 var invalidRules = document.styleSheets[1].cssRules;
 result = '';
 for (var i = 0; i < invalidRules.length; ++i) {
     var itemText = invalidRules.item(i).cssText.replace(/file:.*LayoutTests\/fast\/css\//, '');
     result += itemText + '\n';
 }
 document.getElementById('invalidResult').innerText = result;
 </script>