(Temporarily?) disable building JS targets on mac hosts

We can't currently build V8 on a mac host targetting Android since the
v8 snapshot creation step requires a 32 bit host toolchain to target a
32 bit target and we don't have a 32 bit host toolchain configured (yet).
Disable for now.

R=cstout@chromium.org

Review URL: https://codereview.chromium.org/1127273010
diff --git a/services/BUILD.gn b/services/BUILD.gn
index 81c59ab..ef45031 100644
--- a/services/BUILD.gn
+++ b/services/BUILD.gn
@@ -13,7 +13,6 @@
     "//services/gles2:lib",
     "//services/http_server",
     "//services/icu_data",
-    "//services/js",
     "//services/kiosk_wm",
     "//services/native_viewport",
     "//services/reaper",
@@ -37,4 +36,10 @@
   if (is_linux) {
     deps += [ "//services/python" ]
   }
+
+  # TODO(jamesr): We only support building V8 snapshot data on a linux host since it
+  # needs a 32 bit toolchain and we don't have one configured for mac hosts.
+  if (host_os == "linux") {
+    deps += [ "//services/js" ]
+  }
 }