Add support for tagging layers.

cc/layer_tree_host.cc

Index: cc/layer_tree_host.cc
diff --git a/cc/layer_tree_host.cc b/cc/layer_tree_host.cc
index 91276deb8ad5bcb005ec733ce1713e3e0f543d63..ca6d2633b517b429d1a32482b863e6a93d01c7e2 100644
--- a/cc/layer_tree_host.cc
+++ b/cc/layer_tree_host.cc
@@ -900,4 +900,28 @@ void LayerTreeHost::setAnimationEventsRecursive(const AnimationEventsVector& eve
         setAnimationEventsRecursive(events, layer->children()[childIndex].get(), wallClockTime);
 }
 
+Layer* LayerTreeHost::getFirstLayerWithTags(unsigned int tags) const
+{
+    std::queue<Layer*> layers;
+    layers.push(m_rootLayer.get());
+
+    while (!layers.empty()) {
+        Layer* layer = layers.front();
+        layers.pop();
+
+        if (!layer)
+            return NULL;
+
+        if (layer->getTags() & tags)
+            return layer;
+
+        for (LayerList::const_iterator it = layer->children().begin();
+             it != layer->children().end(); ++it) {
+            layers.push(it->get());
+        }
+    }
+
+    return NULL;
+}
+
 }  // namespace cc