Add gn dependency on v8 back in

BUG=none
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/265403004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271888 0039d316-1c4b-4281-b951-d872f2087c98

BUILD.gn

@@ -68,6 +68,7 @@ group("root") {
     "//ui/strings",
     "//ui/surface",
     "//url",
+    "//v8:mksnapshot",
   ]
 
   if (is_linux) {