[node] move build path fully into gyp file.

R=machenbach@chromium.org

Bug: v8:6105
Change-Id: Ieb0de4d0ee031d61d7af1a89607bfbcb64151bcb
Reviewed-on: https://chromium-review.googlesource.com/874353Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50705}

src/v8.gyp

@@ -2611,7 +2611,7 @@
             '../tools/node/build_gn.py',
             '--mode', '<(CONFIGURATION_NAME)',
             '--v8_path', '../',
-            '--build_path', '<(INTERMEDIATE_DIR)',
+            '--build_path', '<(INTERMEDIATE_DIR)/gn',
             '--host_os', '<(host_os)',
             '--flag', 'v8_promise_internal_field_count=<(v8_promise_internal_field_count)',
             '--flag', 'target_cpu="<(target_arch)"',

tools/node/build_gn.py

@@ -34,8 +34,6 @@ GN_ARGS = [
 
 BUILD_TARGET = "v8_monolith"
 
-BUILD_SUBDIR = "gn"
-
 def FindGn(options):
   if options.host_os == "linux":
     os_path = "linux64"
@@ -96,7 +94,6 @@ def ParseOptions(args):
 
   assert options.build_path
   options.build_path = os.path.abspath(options.build_path)
-  options.build_path = os.path.join(options.build_path, BUILD_SUBDIR)
   return options
 
 if __name__ == "__main__":