Use parent-relative coordinates in AXTreeSourceAura

Convert the global coordinates reported by each AXAuraObjWrapper into
parent-relative coordinates to be used in the accessibility tree. That
way when any Window, Widget, or View moves (and fires a location
changed event), its descendants all move relative to it by default.

Bug: 761164
Change-Id: I19eae6f1ca5f00726107aca764d537d776cc16f3
Reviewed-on: https://chromium-review.googlesource.com/780362Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518450}

chrome/browser/ui/aura/accessibility/ax_tree_source_aura.cc

@@ -108,6 +108,19 @@ void AXTreeSourceAura::SerializeNode(AXAuraObjWrapper* node,
                                      ui::AXNodeData* out_data) const {
   node->Serialize(out_data);
 
+  // Convert the global coordinates reported by each AXAuraObjWrapper
+  // into parent-relative coordinates to be used in the accessibility
+  // tree. That way when any Window, Widget, or View moves (and fires
+  // a location changed event), its descendants all move relative to
+  // it by default.
+  AXAuraObjWrapper* parent = node->GetParent();
+  if (parent) {
+    ui::AXNodeData parent_data;
+    parent->Serialize(&parent_data);
+    out_data->location.Offset(-parent_data.location.OffsetFromOrigin());
+    out_data->offset_container_id = parent->GetID();
+  }
+
   if (out_data->role == ui::AX_ROLE_WEB_VIEW) {
     views::View* view = static_cast<views::AXViewObjWrapper*>(node)->view();
     content::WebContents* contents =