Make Document::caretRangeFromPoint() to work with :first-letter style

LayoutTests/fast/dom/Document/CaretRangeFromPoint/caretRangeFromPoint-with-first-letter-style.html

+<style>
+p.first-letter:first-letter { color: red; }
+</style>
+<div id="container">
+<p id="description"></p>
+<p id="sample" class="first-letter"><span>0123456</span></p>
+</div>
+<div id="console"></div>
+<script src="../../../../fast/js/resources/js-test-pre.js"></script>
+<script>
+description('caretRangeFromPoint() should work with :first-letter style');
+function $(id) { return document.getElementById(id); }
+
+var middle = $('sample').offsetTop + $('sample').offsetHeight / 2;
+var left = $('sample').offsetLeft;
+var numberOfChars = $('sample').firstChild.firstChild.length;
+var charWidth = $('sample').firstChild.offsetWidth / numberOfChars;
+var x = left;
+for (var i = 0; i < numberOfChars; ++i) {
+    var range = document.caretRangeFromPoint(x + 3, middle);
+    shouldBe('range.startOffset', i.toString());
+    x += charWidth;
+}
+
+if (window.testRunner)
+    $('container').outerHTML = '';
+</script>
+<script src="../../../../fast/js/resources/js-test-post.js"></script>