Windows precompiled header support in GN

tools/gn/tool.h

Index: tools/gn/tool.h
diff --git a/tools/gn/tool.h b/tools/gn/tool.h
index 14a64c1b6df661a353ace812f4a4bee006ab3057..9ccd56fd8daf8a021a618381fbe8dc23bf6ee9d0 100644
--- a/tools/gn/tool.h
+++ b/tools/gn/tool.h
@@ -19,6 +19,12 @@ class Tool {
     DEPS_MSVC = 1
   };
 
+  enum PrecompiledHeaderType {
+    PCH_NONE = 0,
+    PCH_GCC = 1,
+    PCH_MSVC = 2
+  };
+
   Tool();
   ~Tool();
 
@@ -63,6 +69,13 @@ class Tool {
     depsformat_ = f;
   }
 
+  PrecompiledHeaderType precompiled_header_type() const { 
+    return precompiled_header_type_;
+  }
+  void set_precompiled_header_type(PrecompiledHeaderType pch_type) {
+    precompiled_header_type_ = pch_type;
+  }
+
   const SubstitutionPattern& description() const {
     return description_;
   }
@@ -167,6 +180,7 @@ class Tool {
   std::string default_output_extension_;
   SubstitutionPattern depfile_;
   DepsFormat depsformat_;
+  PrecompiledHeaderType precompiled_header_type_;
   SubstitutionPattern description_;
   std::string lib_switch_;
   std::string lib_dir_switch_;