Add GN substitution support for Ninja's $in_newline

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 4f99778139851c44591edd024722f76d20e1dddf..a0bb9a909307db38d9c3212215f863a255dbd054 100644
--- a/tools/gn/function_toolchain.cc
+++ b/tools/gn/function_toolchain.cc
@@ -575,10 +575,17 @@ const char kTool_Help[] =
     "  The following expansions are available:\n"
     "\n"
     "    {{inputs}}\n"
+    "    {{inputs_newline}}\n"
     "        Expands to the inputs to the link step. This will be a list of\n"
     "        object files and static libraries.\n"
     "        Example: \"obj/foo.o obj/bar.o obj/somelibrary.a\"\n"
     "\n"
+    "        The \"_newline\" version will separate the input files with\n"
+    "        newlines instead of spaces. This is useful in response files:\n"
+    "        some linkers can take a \"-filelist\" flag which expects newline\n"
+    "        separated files, and some Microsoft tools have a fixed-sized\n"
+    "        buffer for parsing each line of a response file.\n"
+    "\n"
     "    {{ldflags}}\n"
     "        Expands to the processed set of ldflags and library search paths\n"
     "        specified for the target.\n"