Creates AXPosition to uniquely identify a position in the accessibility tree

ui/accessibility/BUILD.gn

Index: ui/accessibility/BUILD.gn
diff --git a/ui/accessibility/BUILD.gn b/ui/accessibility/BUILD.gn
index 4c03ce91d34e55a5f9e7465509fafdb339a30adc..1fc5e7be9954ec81f9ff5362c05cc5faf7765738 100644
--- a/ui/accessibility/BUILD.gn
+++ b/ui/accessibility/BUILD.gn
@@ -17,6 +17,9 @@ component("accessibility") {
     "ax_node.h",
     "ax_node_data.cc",
     "ax_node_data.h",
+    "ax_node_position.cc",
+    "ax_node_position.h",
+    "ax_position.h",
     "ax_relative_bounds.cc",
     "ax_relative_bounds.h",
     "ax_serializable_tree.cc",
@@ -119,6 +122,7 @@ static_library("test_support") {
 test("accessibility_unittests") {
   sources = [
     "ax_generated_tree_unittest.cc",
+    "ax_node_position_unittest.cc",
     "ax_text_utils_unittest.cc",
     "ax_tree_combiner_unittest.cc",
     "ax_tree_serializer_unittest.cc",