GN: Use build directory for CD for scripts

tools/gn/functions_target.cc

Index: tools/gn/functions_target.cc
diff --git a/tools/gn/functions_target.cc b/tools/gn/functions_target.cc
index 7e046019964914acb350a5e708c70694bd204540..3d1631105a5dbe61131c9cdaeb59ebac2c884296 100644
--- a/tools/gn/functions_target.cc
+++ b/tools/gn/functions_target.cc
@@ -123,7 +123,11 @@ const char kCustom_Help[] =
     "  files and generate a set of output files.\n"
     "\n"
     "  The script will be executed with the given arguments with the current\n"
-    "  directory being that of the current BUILD file.\n"
+    "  directory being that of the root build directory. If you pass\n"
+    "  to your script, see \"gn help to_build_path\" for how to convert\n"
+    "  file names to be relative to the build directory (file names in the\n"
+    "  sources, outputs, and source_prereqs will be all treated as relative\n"
+    "  to the current build file and converted as needed automatically).\n"
     "\n"
     "  There are two modes. The first mode is the \"per-file\" mode where you\n"
     "  specify a list of sources and the script is run once for each one as a\n"
@@ -151,8 +155,8 @@ const char kCustom_Help[] =
     "\n"
     "    {{source}}\n"
     "        Expanded in |args|, this is the name of the source file relative\n"
-    "        to the current directory when running the script. This is how\n"
-    "        you specify the current input file to your script.\n"
+    "        to the build directory This is how you specify the current input\n"
+    "        file to your script.\n"
     "\n"
     "    {{source_name_part}}\n"
     "        Expanded in |args| and |outputs|, this is just the filename part\n"
@@ -176,16 +180,21 @@ const char kCustom_Help[] =
     "    source_prereqs = [ \"my_configuration.txt\" ]\n"
     "    outputs = [ \"$target_gen_dir/{{source_name_part}}.h\",\n"
     "                \"$target_gen_dir/{{source_name_part}}.cc\" ]\n"
+    "\n"
+    "    # Note that since \"args\" is opaque to GN, if you specify paths\n"
+    "    # here, you will need to convert it to be relative to the build\n"
+    "    # directory using \"to_build_path()\".\n"
     "    args = [ \"{{source}}\",\n"
     "             \"-o\",\n"
-    "             \"$relative_target_gen_dir/{{source_name_part}}.h\" ]\n"
+    "             to_build_path(relative_target_gen_dir) + \"/\" +\n"
+    "                 {{source_name_part}}.h\" ]\n"
     "  }\n"
     "\n"
     "  custom(\"just_run_this_guy_once\") {\n"
     "    script = \"doprocessing.py\"\n"
     "    source_prereqs = [ \"my_configuration.txt\" ]\n"
     "    outputs = [ \"$target_gen_dir/insightful_output.txt\" ]\n"
-    "    args = [ \"--output_dir\", $target_gen_dir ]\n"
+    "    args = [ \"--output_dir\", to_build_path(target_gen_dir) ]\n"
     "  }\n";
 
 Value RunCustom(Scope* scope,