Add Unicode Variation Selector support to harfBuzzGetGlyph

LayoutTests/fast/text/unicode-variation-selector.html

 <style>
+.ahem {
+    font:20px/1 Ahem;
+}
 @font-face {
     font-family: ivsfont;
     src: url(resources/AhemIVS.ttf);
 }
 .ivs {
-    font-family: ivsfont;
+    font:20px/1 ivsfont;
 }
 </style>
 <p>
 This page ensures that WebKit can render unicode variation selector correctly.  On platforms which support UVSes, the glyph of U+0061 should be a 0.2em high, 1em wide rectangle. On platforms which don't support UVSes, it should be a square.  In addition, any glyphs (including the last resort glyph) should not appear after the glyph on all platforms.
 </p>
 <div>
-Glyph for code point U+0061 without UVS:
-<span style="font-family: ivsfont;">&#x0061;</span>
+Without UVS:
+<span class="ivs">&#x0061;</span>
+should look like <span class="ahem">X</span>
 </div>
 <div>
-Glyph for code point U+0061 with UVS:
-<span style="font-family: ivsfont;" id='ivs-holder'>&#x0061;&#xE0100;</span>
+With UVS:
+<span class="ivs">&#x0061;&#xE0100;</span>
+should look like <span class="ahem">p</span>
 </div>
+<div>
+UVS not in the font should fallback to base:
+<span class="ivs">&#x0061;&#xE0101;</span> should look like <span class="ahem">X</span>
+</div>
+<script>
+if (window.testRunner) {
+    testRunner.waitUntilDone();
+    document.fonts.ready.then(function () {
+        testRunner.notifyDone();
+    });
+}
+</script>