Hook up symbol files to the Windows GN build.

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 b3821eca774e0c1a7bf4d0e9ed3ae78fd20550c2..377b92720ad26087819d50d3fd574e47ced9bcab 100644
--- a/tools/gn/function_toolchain.cc
+++ b/tools/gn/function_toolchain.cc
@@ -160,13 +160,14 @@ const char kTool_Help[] =
     "  given file type. See also \"gn help toolchain\".\n"
     "\n"
     "Command types:\n"
+    "\n"
     "  The following values may be passed to the tool() function for the type\n"
     "  of the command:\n"
     "\n"
     "  \"cc\", \"cxx\", \"objc\", \"objcxx\", \"asm\", \"alink\", \"solink\",\n"
     "  \"link\", \"stamp\", \"copy\"\n"
     "\n"
-    "Command flags:\n"
+    "Command flags\n"
     "\n"
     "  These variables may be specified in the { } block after the tool call.\n"
     "  They are passed directly to Ninja. See the ninja documentation for how\n"
@@ -188,7 +189,37 @@ const char kTool_Help[] =
     "  added to the link like with a \"-framework\" switch and the lib prefix\n"
     "  will be ignored.\n"
     "\n"
-    "Example:\n"
+    "Ninja variables available to tool invocations\n"
+    "\n"
+    "  When writing tool commands, you use the various built-in Ninja\n"
+    "  variables like \"$in\" and \"$out\" (note that the $ must be escaped\n"
+    "  for it to be passed to Ninja, so write \"\\$in\" in the command\n"
+    "  string).\n"
+    "\n"
+    "  GN defines the following variables for binary targets to access the\n"
+    "  various computed information needed for compiling:\n"
+    "\n"
+    "    - Compiler flags: \"cflags\", \"cflags_c\", \"cflags_cc\",\n"
+    "          \"cflags_objc\", \"cflags_objcc\"\n"
+    "\n"
+    "    - Linker flags: \"ldflags\", \"libs\"\n"
+    "\n"
+    "  GN sets these other variables with target information that can be\n"
+    "  used for computing names for supplimetary files:\n"
+    "\n"
+    "    - \"target_name\": The name of the current target with no\n"
+    "      path information. For example \"mylib\".\n"
+    "\n"
+    "    - \"target_out_dir\": The value of \"target_out_dir\" from the BUILD\n"
+    "      file for this target (see \"gn help target_out_dir\"), relative\n"
+    "      to the root build directory with no trailing slash.\n"
+    "\n"
+    "    - \"root_out_dir\": The value of \"root_out_dir\" from the BUILD\n"
+    "      file for this target (see \"gn help root_out_dir\"), relative\n"
+    "      to the root build directory with no trailing slash.\n"
+    "\n"
+    "Example\n"
+    "\n"
     "  toolchain(\"my_toolchain\") {\n"
     "    # Put these at the top to apply to all tools below.\n"
     "    lib_prefix = \"-l\"\n"