Fix BUILD.gn after unflagging unscopables

TBR=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/455823002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23001 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

BUILD.gn

@@ -242,7 +242,6 @@ action("js2c_experimental") {
     "src/generator.js",
     "src/harmony-string.js",
     "src/harmony-array.js",
-    "src/unscopables.js",
   ]
 
   outputs = [