blink: Add backdrop-filter support

LayoutTests/css3/filters/backdrop-filter-rendering.html

+<script>
+if (window.testRunner)
+    testRunner.waitUntilDone();
+
+window.addEventListener('load', function () {
+    var el = document.querySelector("div");
+    el.className = "backdrop_filter";
+    setTimeout(function () {
+        if (window.testRunner)
+            testRunner.notifyDone();

[jbroman, 2015/08/20 02:49:46]

Drive-by: please no setTimeout in layout tests. You probably want
testRunner.layoutAndPaintAsyncThen, or runAfterLayoutAndPaint, which wraps it.

(That is, if you can't just make the test ready in time. It's not clear to me
why you're changing the className of the first element to also be
backdrop_filter, but I'll leave that to your reviewer.)

And, the obligatory: could this be a ref test?

+    }, 0);
+}, false);
+</script>
+<style>
+div {
+  width: 100px;
+  height: 100px;
+  position: absolute;
+}
+
+.backdrop_object {
+  left: 120px;
+  top: 80px;
+  background: orange;
+}
+
+.backdrop_filter {
+  left: 70px;
+  top: 50px;
+  background: blue;
+  opacity:0.25;
+  backdrop-filter: blur(5px);
+}
+</style>
+
+<div class="backdrop_object"></div>
+<div class="backdrop_filter"></div>