Adding a performance test for font fallback

PerformanceTests/Layout/resources/character_fallback_test.js

+// Perf Tests run a maximum of 20 times,
+// make sure we have an equal amount of characters
+// for each run.
+var selectionSize = fallbackChars.length / 21;
+var target;
+
+function test() {
+    var charSelection = "";
+    for(var i=0; i < selectionSize; i++) {
+        var selectedCharIndex = Math.floor(Math.random() * fallbackChars.length);
+        if(!fallbackChars[selectedCharIndex])
+            continue;
+        charSelection += fallbackChars[selectedCharIndex];
+        fallbackChars.splice(selectedCharIndex, 1);
+    }
+    if (charSelection.length)
+        replaceTextAndWaitForLayout(charSelection);
+}
+
+function replaceTextAndWaitForLayout(charSelection) {
+    while (target.firstChild)
+        target.removeChild(target.firstChild);
+    target.appendChild(document.createTextNode(charSelection));
+    target.offsetHeight;
+}
+
+function cleanup() {
+    replaceTextAndWaitForLayout("");
+}
+
+function startTest() {
+    target = document.querySelector("#target");
+    PerfTestRunner.measureTime({ run: test, done: cleanup, description: "Per-character font fallback" });
+}