Eliminate child_count()/GetChildAtIndex() from AXNode.

This also makes a slew of related changes along the way, mostly by switching
various types to size_t.

Bug: 956419
Change-Id: I7e774ee70315a3af632a780fc2f7fa17c96b891a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618269
Reviewed-by: Lei Zhang <[email protected]>
Reviewed-by: Aaron Leventhal <[email protected]>
Commit-Queue: Peter Kasting <[email protected]>
Cr-Commit-Position: refs/heads/master@{#662309}
diff --git a/ui/accessibility/ax_tree.h b/ui/accessibility/ax_tree.h
index a151d76..afc6982 100644
--- a/ui/accessibility/ax_tree.h
+++ b/ui/accessibility/ax_tree.h
@@ -170,7 +170,7 @@
 
   AXNode* CreateNode(AXNode* parent,
                      int32_t id,
-                     int32_t index_in_parent,
+                     size_t index_in_parent,
                      AXTreeUpdateState* update_state);
 
   // This is called from within Unserialize(), it returns true on success.