Changed loop var type from size_t to int

The problem was here:

for (size_t i = children().size() - 1; i >= 0; --i) {

The loop variable |size_t i| was of an unsigned int type, so |i >= 0|
was always true.

Repro on Windows by exploring the tree with Inspect on the link provided
in the crbug description.

Bug: 988153
Change-Id: I1cae093da68f12ed1630a568ec7529703c73cf89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1737870
Commit-Queue: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684428}

ui/accessibility/ax_node.cc

@@ -772,7 +772,7 @@ AXNode* AXNode::ComputeLastUnignoredChildRecursive() const {
   if (children().size() == 0)
     return nullptr;
 
-  for (size_t i = children().size() - 1; i >= 0; --i) {
+  for (int i = static_cast<int>(children().size()) - 1; i >= 0; --i) {
     AXNode* child = children_[i];
     if (!child->data().HasState(ax::mojom::State::kIgnored))
       return child;

ui/accessibility/ax_tree_unittest.cc

@@ -1871,10 +1871,10 @@ TEST(AXTreeTest, UnignoredAccessors) {
   //     |      |____
   //     |      |   |
   //     13(i)  14  15
-  //     |
-  //     16
+  //     |      |
+  //     16     17
   tree_update.root_id = 1;
-  tree_update.nodes.resize(16);
+  tree_update.nodes.resize(17);
   tree_update.nodes[0].id = 1;
   tree_update.nodes[0].child_ids = {2, 3, 4};
 
@@ -1915,11 +1915,15 @@ TEST(AXTreeTest, UnignoredAccessors) {
   tree_update.nodes[12].AddState(ax::mojom::State::kIgnored);
 
   tree_update.nodes[13].id = 14;
+  tree_update.nodes[13].child_ids = {17};
 
   tree_update.nodes[14].id = 15;
 
   tree_update.nodes[15].id = 16;
 
+  tree_update.nodes[16].id = 17;
+  tree_update.nodes[16].AddState(ax::mojom::State::kIgnored);
+
   AXTree tree(tree_update);
 
   EXPECT_EQ(4, tree.GetFromId(1)->GetLastUnignoredChild()->id());
@@ -1938,6 +1942,7 @@ TEST(AXTreeTest, UnignoredAccessors) {
   EXPECT_EQ(nullptr, tree.GetFromId(14)->GetLastUnignoredChild());
   EXPECT_EQ(nullptr, tree.GetFromId(15)->GetLastUnignoredChild());
   EXPECT_EQ(nullptr, tree.GetFromId(16)->GetLastUnignoredChild());
+  EXPECT_EQ(nullptr, tree.GetFromId(17)->GetLastUnignoredChild());
 }
 
 TEST(AXTreeTest, UnignoredNextPreviousChild) {