Support spaces in Mac GN build output names.

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 7068370e49f6555862370fe4405949cc283624dc..ed4bbc37d96105428eb47585422fbb078c075382 100644
--- a/tools/gn/function_toolchain.cc
+++ b/tools/gn/function_toolchain.cc
@@ -595,6 +595,13 @@ const char kTool_Help[] =
     "        omitted from the label for targets in the default toolchain, and\n"
     "        will be included for targets in other toolchains.\n"
     "\n"
+    "    {{label_name}}\n"
+    "        The short name of the label of the target. This is the part\n"
+    "        after the colon. For \"//foo/bar:baz\" this will be \"baz\".\n"
+    "        Unlike {{target_output_name}}, this is not affected by the\n"
+    "        \"output_prefix\" in the tool or the \"output_name\" set\n"
+    "        on the target.\n"
+    "\n"
     "    {{output}}\n"
     "        The relative path and name of the output(s) of the current\n"
     "        build step. If there is more than one output, this will expand\n"
@@ -612,6 +619,7 @@ const char kTool_Help[] =
     "        The short name of the current target with no path information,\n"
     "        or the value of the \"output_name\" variable if one is specified\n"
     "        in the target. This will include the \"output_prefix\" if any.\n"
+    "        See also {{label_name}}.\n"
     "        Example: \"libfoo\" for the target named \"foo\" and an\n"
     "        output prefix for the linker tool of \"lib\".\n"
     "\n"