Windows precompiled header support in GN

tools/gn/function_toolchain.cc

Index: tools/gn/function_toolchain.cc
diff --git a/tools/gn/function_toolchain.cc b/tools/gn/function_toolchain.cc
index c4a70373682ff55e23945c85bf50fa55eb9ab93c..289d0d245f739b3ef388f87ef00b82fdf8686316 100644
--- a/tools/gn/function_toolchain.cc
+++ b/tools/gn/function_toolchain.cc
@@ -503,6 +503,20 @@ const char kTool_Help[] =
     "        Posix systems:\n"
     "          output_prefix = \"lib\"\n"
     "\n"
+    "    precompiled_header_type  [string]\n"
+    "        Valid for: \"cc\", \"cxx\", \"objc\", \"objcxx\"\n"
+    "\n"
+    "        Type of precompiled headers. If undefined or the empty string,\n"
+    "        precompiled headers will not be used for this tool. Otherwise\n"
+    "        use either \"gcc\" or \"msvc\".\n"
+    "\n"
+    "        For precompiled headers to be used for a given target, the\n"
+    "        target (or a config applied to it) must also specify a\n"
+    "        \"precompiled_header\" and, for \"msvc\"-style headers, a\n"
+    "        \"precompiled_source\" value.\n"
+    "\n"
+    "        See \"gn help precompiled_header\" for more.\n"
+    "\n"
     "    restat  [boolean]\n"
     "        Valid for: all tools (optional, defaults to false)\n"
     "\n"
@@ -540,7 +554,7 @@ const char kTool_Help[] =
     "            rspfile_content = \"{{inputs}} {{solibs}} {{libs}}\"\n"
     "          }\n"
     "\n"
-    "Expansions for tool variables"
+    "Expansions for tool variables\n"
     "\n"
     "  All paths are relative to the root build directory, which is the\n"
     "  current directory for running all tools. These expansions are\n"