GN Command line -D parsing, goma on Windows

build/toolchain/win/BUILD.gn

Index: build/toolchain/win/BUILD.gn
diff --git a/build/toolchain/win/BUILD.gn b/build/toolchain/win/BUILD.gn
index 6abf8082daac2d2e23fe8cb399bf5a94ef728427..ceb0ca8d5c78ac872c598a1739fa20e3ed04d631 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/toolchain/goma.gni")
+
 # Should only be running on Windows.
 assert(is_win)
 
@@ -26,6 +28,11 @@ exec_script("setup_toolchain.py", [ gyp_win_tool_source ], "value")
 stamp_command = "$python_path gyp-win-tool stamp \$out"
 copy_command = "$python_path gyp-win-tool recursive-mirror \$in \$out"
 
+if (use_goma) {
+  # Add the Goma compiler prefixes to the GYP file.
+  gyp_header = make_goma_global_settings
+}
+
 # 32-bit toolchain -------------------------------------------------------------
 
 toolchain("32") {