Remove 'String::append' from 'Source/core/accessibility/AccessibilityNodeObject.cpp'.

Replaced with StringBuilder for (marginally) better performance.

BUG=268281

Review URL: https://chromiumcodereview.appspot.com/22042003

git-svn-id: svn://svn.chromium.org/blink/trunk@155524 bbb929c8-8fbe-4397-9dbb-9b2b20218538