accessibility: Simpler code using ChildAtIndex() method.

ChildAtIndex() is method of AxNode that does the same thing of
children()[index], but reads better.

BUG=None
TEST=content_unittests
R=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/919343006

Cr-Commit-Position: refs/heads/master@{#316392}

content/browser/accessibility/browser_accessibility.cc

@@ -130,7 +130,7 @@ BrowserAccessibility* BrowserAccessibility::InternalGetChild(
     uint32 child_index) const {
   if (!node_ || !manager_)
     return NULL;
-  return manager_->GetFromAXNode(node_->children()[child_index]);
+  return manager_->GetFromAXNode(node_->ChildAtIndex(child_index));
 }
 
 BrowserAccessibility* BrowserAccessibility::GetParent() const {