libstdc++: fix incomplete-type in AXTree for NodeSetSizePosInSetInfo

has only forward declaration of NodeSetSizePosInSetInfo. Therefore,
move declaration from ax_tree.cc.

std: :unordered_map<T, U> requires U to be fully declared. ax_tree.h
Bug: 957519
Change-Id: Ic1f4bf3ebfea229ece84251e46d4461b31873868
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2132403Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760588}

ui/accessibility/ax_tree.cc

@@ -567,14 +567,8 @@ struct AXTreeUpdateState {
   const AXTree& tree;
 };
 
-struct AXTree::NodeSetSizePosInSetInfo {
-  NodeSetSizePosInSetInfo() = default;
-  ~NodeSetSizePosInSetInfo() = default;
-
-  int32_t pos_in_set = 0;
-  int32_t set_size = 0;
-  base::Optional<int> lowest_hierarchical_level;
-};
+AXTree::NodeSetSizePosInSetInfo::NodeSetSizePosInSetInfo() = default;
+AXTree::NodeSetSizePosInSetInfo::~NodeSetSizePosInSetInfo() = default;
 
 struct AXTree::OrderedSetContent {
   explicit OrderedSetContent(const AXNode* ordered_set = nullptr)

ui/accessibility/ax_tree.h

@@ -328,7 +328,14 @@ class AX_EXPORT AXTree : public AXNode::OwnerTree {
   bool enable_extra_mac_nodes_ = false;
 
   // Contains pos_in_set and set_size data for an AXNode.
-  struct NodeSetSizePosInSetInfo;
+  struct NodeSetSizePosInSetInfo {
+    NodeSetSizePosInSetInfo();
+    ~NodeSetSizePosInSetInfo();
+
+    int32_t pos_in_set = 0;
+    int32_t set_size = 0;
+    base::Optional<int> lowest_hierarchical_level;
+  };
 
   // Represents the content of an ordered set which includes the ordered set
   // items and the ordered set container if it exists.