[GN] Uses the consistent way to disallow assign and copy

The code has several places where the manual disallowing
of copy and assignment is used. It stands out from style
and may lead to a compilation error, if you're building
under a specific platform/compiler.

R=sdefresne@chromium.org

Bug: None
Change-Id: I6df0fec2d575675565a78e63ea8efab854c8a8f6
Reviewed-on: https://chromium-review.googlesource.com/690000Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505997}

tools/gn/build_settings.h

@@ -125,7 +125,7 @@ class BuildSettings {
 
   std::unique_ptr<std::set<SourceFile>> exec_script_whitelist_;
 
-  BuildSettings& operator=(const BuildSettings& other);  // Disallow.
+  DISALLOW_ASSIGN(BuildSettings);
 };
 
 #endif  // TOOLS_GN_BUILD_SETTINGS_H_

tools/gn/pool.h

@@ -7,6 +7,7 @@
 
 #include <string>
 
+#include "base/macros.h"
 #include "tools/gn/item.h"
 
 // Represents a named pool in the dependency graph.
@@ -18,9 +19,6 @@ class Pool : public Item {
   using Item::Item;
   ~Pool() override;
 
-  Pool(const Pool&) = delete;
-  Pool& operator=(const Pool&) = delete;
-
   // Item implementation.
   Pool* AsPool() override;
   const Pool* AsPool() const override;
@@ -36,6 +34,8 @@ class Pool : public Item {
   std::string GetNinjaName(bool include_toolchain) const;
 
   int64_t depth_ = 0;
+
+  DISALLOW_COPY_AND_ASSIGN(Pool);
 };
 
 #endif  // TOOLS_GN_POOL_H_