Allow inherited constructors and add a sample usage.

This was OKed on the discussion thread back in July, but never added.

BUG=none
TEST=none

Review-Url: https://codereview.chromium.org/2468633003
Cr-Original-Commit-Position: refs/heads/master@{#429716}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3e35c53fdc3e68aa70efbca7f49a8494df79db1b

tools/gn/pool.cc

diff --git a/tools/gn/pool.cc b/tools/gn/pool.cc
index e4fc206..75a7504 100644
--- a/tools/gn/pool.cc
+++ b/tools/gn/pool.cc
@@ -8,9 +8,6 @@
 
 #include "base/logging.h"
 
-Pool::Pool(const Settings* settings, const Label& label)
-    : Item(settings, label) {}
-
 Pool::~Pool() {}
 
 Pool* Pool::AsPool() {

tools/gn/pool.h

diff --git a/tools/gn/pool.h b/tools/gn/pool.h
index d719364..81a021d 100644
--- a/tools/gn/pool.h
+++ b/tools/gn/pool.h
@@ -15,7 +15,7 @@
 // generated ninja build. Pools are referenced by toolchains.
 class Pool : public Item {
  public:
-  Pool(const Settings* settings, const Label& label);
+  using Item::Item;
   ~Pool() override;
 
   Pool(const Pool&) = delete;