Fix crash when AXTreeCombiner is called with an empty tree.

BUG=727290

Change-Id: Id54681bca8dce8b6a19d40c713a397a43e761d33
Reviewed-on: https://chromium-review.googlesource.com/527912Reviewed-by: Paul Miller <paulmiller@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#480288}

ui/accessibility/ax_tree_combiner.cc

@@ -40,7 +40,7 @@ bool AXTreeCombiner::Combine() {
   ProcessTree(root);
 
   // Set the root id.
-  combined_.root_id = combined_.nodes[0].id;
+  combined_.root_id = combined_.nodes.size() > 0 ? combined_.nodes[0].id : 0;
 
   // Finally, handle the tree ID, taking into account which subtree might
   // have focus and mapping IDs from the tree data appropriately.

ui/accessibility/ax_tree_combiner_unittest.cc

@@ -192,4 +192,15 @@ TEST(CombineAXTreesTest, FocusedTree) {
   EXPECT_EQ(6, combined.tree_data.focus_id);
 }
 
+TEST(CombineAXTreesTest, EmptyTree) {
+  AXTreeUpdate tree;
+
+  AXTreeCombiner combiner;
+  combiner.AddTree(tree, true);
+  combiner.Combine();
+
+  const AXTreeUpdate& combined = combiner.combined();
+  ASSERT_EQ(0U, combined.nodes.size());
+}
+
 }  // namespace ui