Introduce mkgrokdump to update tools/v8heapconst.py.

test/mkgrokdump/BUILD.gn

Index: test/mkgrokdump/BUILD.gn
diff --git a/test/mkgrokdump/BUILD.gn b/test/mkgrokdump/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..ce3587ba3501b2a2f7c963f3ebb2ebd74854a4fa
--- /dev/null
+++ b/test/mkgrokdump/BUILD.gn
@@ -0,0 +1,26 @@
+# Copyright 2017 the V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("../../gni/v8.gni")
+
+v8_executable("mkgrokdump") {
+  # mkgrokdump is used to create tools/v8heapconst.py.
+
+  sources = [
+    "mkgrokdump.cc",
+  ]
+
+  configs = [
+    "../..:external_config",
+    "../..:internal_config",
+  ]
+
+  deps = [
+    "../..:v8",
+    "../..:v8_libbase",
+    "../..:v8_libplatform",
+    "//build/config/sanitizers:deps",
+    "//build/win:default_exe_manifest",
+  ]
+}