Make goma work on win GN builds.

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 87bd254ca8303b4a04e3cb59965ec34e1b9fa2a2..517d192033ee4892e424225185f5c6d95cd8339e 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -46,6 +46,14 @@ template("msvc_toolchain") {
                                         configuration,
                                         invoker.cpu_arch])
 
+  if (use_goma) {
+    goma_prefix = "$goma_dir/gomacc.exe "
+  } else {
+    goma_prefix = ""
+  }
+
+  cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
+
   toolchain(target_name) {
     # Make these apply to all tools below.
     lib_switch = ""
@@ -54,7 +62,7 @@ template("msvc_toolchain") {
     tool("cc") {
       rspfile = "{{output}}.rsp"
       pdbname = "{{target_out_dir}}/{{target_output_name}}_c.pdb"
-      command = "ninja -t msvc -e $env -- cl.exe /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
+      command = "ninja -t msvc -e $env -- $cl /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
       depsformat = "msvc"
       description = "CC {{output}}"
       outputs = [
@@ -67,7 +75,7 @@ template("msvc_toolchain") {
       rspfile = "{{output}}.rsp"
       # The PDB name needs to be different between C and C++ compiled files.
       pdbname = "{{target_out_dir}}/{{target_output_name}}_cc.pdb"
-      command = "ninja -t msvc -e $env -- cl.exe /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
+      command = "ninja -t msvc -e $env -- $cl /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
       depsformat = "msvc"
       description = "CXX {{output}}"
       outputs = [