Use auto& instead of auto in the range loop

There are a couple reasons for this change:
1. Avoid copying a struct larger than a pointer.
2. If the pointer in the struct gets replaced by CheckedPtr with
   a non-trivial constructor, the compiler will complain.

See go/miracleptr for more details on the experimental CheckedPtr project.

Bug: 1080832
Change-Id: I520614995dcbae3bdab69a1423d288b2eeea9189
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437991
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811661}

extensions/renderer/api/automation/automation_ax_tree_wrapper.cc

@@ -280,7 +280,7 @@ void AutomationAXTreeWrapper::OnAtomicUpdateFinished(
     bool root_changed,
     const std::vector<ui::AXTreeObserver::Change>& changes) {
   DCHECK_EQ(&tree_, tree);
-  for (const auto change : changes) {
+  for (const auto& change : changes) {
     ui::AXNode* node = change.node;
     switch (change.type) {
       case NODE_CREATED: