Fix gn configuration for node debug builds

Change-Id: Ic942391ba5eca97f09bb5f9b89aef0630094c77c
Reviewed-on: https://chromium-review.googlesource.com/1015100Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52664}

tools/node/build_gn.py

@@ -55,7 +55,7 @@ def GenerateBuildFiles(options):
     flag = flag.replace("=0", "=false")
     flag = flag.replace("target_cpu=ia32", "target_cpu=\"x86\"")
     gn_args.append(flag)
-  if options.mode == "DEBUG":
+  if options.mode == "Debug":
     gn_args.append("is_debug = true")
   else:
     gn_args.append("is_debug = false")