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

tools/gn/loader.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_LOADER_H_
 #define TOOLS_GN_LOADER_H_
 
 #include <map>
 #include <set>
 
@@ skipping 61 matching lines @@
 
 class LoaderImpl : public Loader {
  public:
   // Callback to emulate InputFileManager::AsyncLoadFile.
   typedef base::Callback<bool(const LocationRange&,
                               const BuildSettings*,
                               const SourceFile&,
                               const base::Callback<void(const ParseNode*)>&,
                               Err*)> AsyncLoadFileCallback;
 
-  LoaderImpl(const BuildSettings* build_settings);
+  explicit LoaderImpl(const BuildSettings* build_settings);
 
   // Loader implementation.
   void Load(const SourceFile& file,
             const LocationRange& origin,
             const Label& toolchain_name) override;
   void ToolchainLoaded(const Toolchain* toolchain) override;
   Label GetDefaultToolchain() const override;
   const Settings* GetToolchainSettings(const Label& label) const override;
 
   // Sets the message loop corresponding to the main thread. By default this
@@ skipping 79 matching lines @@
   const BuildSettings* build_settings_;
   Label default_toolchain_label_;
 
   // Records for the build config file loads.
   // Owning pointers.
   typedef std::map<Label, ToolchainRecord*> ToolchainRecordMap;
   ToolchainRecordMap toolchain_records_;
 };
 
 #endif  // TOOLS_GN_LOADER_H_