Improve speed of QueryNodeList::RemoveEmptySubnodes

Reduces complexity from O(n^2) to O(n).  Prior to this commit, we were
performing an O(n) vector::erase for each empty child, say O(n) of
them. Now, we do an amortized constant vector::push_back for each
child to keep, along with O(n) constant-time vector::swap operations.

Bug: 1014388
Change-Id: I4e614463c5ac207dd036876a54e65cca8ce0ed25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863154
Commit-Queue: Dan McArdle <dmcardle@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706175}

components/query_parser/query_parser.cc

@@ -190,17 +190,19 @@ void QueryNodeList::AddChild(std::unique_ptr<QueryNode> node) {
 }
 
 void QueryNodeList::RemoveEmptySubnodes() {
+  QueryNodeVector kept_children;
   for (size_t i = 0; i < children_.size(); ++i) {
-    if (children_[i]->IsWord())
+    if (children_[i]->IsWord()) {
+      kept_children.push_back(std::move(children_[i]));
       continue;
+    }
 
     QueryNodeList* list_node = static_cast<QueryNodeList*>(children_[i].get());
     list_node->RemoveEmptySubnodes();
-    if (list_node->children()->empty()) {
-      children_.erase(children_.begin() + i);
-      --i;
-    }
+    if (!list_node->children()->empty())
+      kept_children.push_back(std::move(children_[i]));
   }
+  children_.swap(kept_children);
 }
 
 int QueryNodeList::AppendToSQLiteQuery(base::string16* query) const {