tools/gn: Fix errors found by Facebook's flint tool.

tools/gn/setup.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TOOLS_GN_SETUP_H_
 #define TOOLS_GN_SETUP_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 192 matching lines @@
 // The way to run both at the same time is:
 //   dependent_setup.RunPreMessageLoop();
 //   main_setup.Run();
 //   dependent_setup.RunPostMessageLoop();
 // so that the main setup executes the message loop, but both are run.
 class DependentSetup : public CommonSetup {
  public:
   // Note: this could be one function that takes a CommonSetup*, but then
   // the compiler can get confused what to call, since it also matches the
   // default copy constructor.
-  DependentSetup(Setup* derive_from);
-  DependentSetup(DependentSetup* derive_from);
+  explicit DependentSetup(Setup* derive_from);
+  explicit DependentSetup(DependentSetup* derive_from);
   ~DependentSetup() override;
 
   // These are the two parts of Run() in the regular setup, not including the
   // call to actually run the message loop.
   void RunPreMessageLoop();
   bool RunPostMessageLoop();
 
   Scheduler* GetScheduler() override;
 
  private:
   Scheduler* scheduler_;
 };
 
 #endif  // TOOLS_GN_SETUP_H_