Support explicit pools in actions

tools/gn/ninja_build_writer.cc

Index: tools/gn/ninja_build_writer.cc
diff --git a/tools/gn/ninja_build_writer.cc b/tools/gn/ninja_build_writer.cc
index f3e9aaf59ac6263f573f4c6b904d0c0f97ea2140..d732d90dc9a0d7aaf2b504e9c75ca9db2e8296bf 100644
--- a/tools/gn/ninja_build_writer.cc
+++ b/tools/gn/ninja_build_writer.cc
@@ -285,6 +285,14 @@ void NinjaBuildWriter::WriteAllPools() {
     }
   }
 
+  for (const Target* target : default_toolchain_targets_) {
+    if (target->output_type() == Target::ACTION) {
+      const LabelPtrPair<Pool>& pool = target->action_values().pool();
+      if (pool.ptr)
+        used_pools.insert(pool.ptr);
+    }
+  }
+
   // Write pools sorted by their name, to make output deterministic.
   std::vector<const Pool*> sorted_pools(used_pools.begin(), used_pools.end());
   auto pool_name = [this](const Pool* pool) {