Enabling Skia / Moterm to build with PNaCl Newlib toolchain

To see moterm in action with PNaCl, run:
mojo/devtools/common/mojo_run --enable-multiprocess \
    "mojo:window_manager mojo:moterm_example_app.pexe"

Although moterm builds (and can run in a limited fashion),
the following fails due to a lack of mgl support.
./out/Debug/mojo_shell --enable-multiprocess \
    ./out/Debug/moterm_apptests.pexe.mojo

BUG=#431
R=mseaborn@chromium.org

Review URL: https://codereview.chromium.org/1525043002 .
diff --git a/apps/BUILD.gn b/apps/BUILD.gn
index d5585a8..770023b 100644
--- a/apps/BUILD.gn
+++ b/apps/BUILD.gn
@@ -2,18 +2,33 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-group("apps") {
+import("//build/config/nacl/config.gni")
+
+# Collection of apps which are currently capable of being built as pexes.
+group("portable_apps") {
   testonly = true
 
   deps = [
-    "//apps/benchmark",
-    "//apps/benchmark:apptests",
     "//apps/moterm",
     "//apps/moterm:apptests",
     "//apps/moterm:gl_helper_test_app",
   ]
+}
+
+group("apps") {
+  testonly = true
+
+  deps = [
+    ":portable_apps",
+    "//apps/benchmark",
+    "//apps/benchmark:apptests",
+  ]
 
   if (is_android) {
     deps += [ "//apps/shortcut" ]
   }
+
+  if (mojo_use_nacl) {
+    deps += [ ":portable_apps(//build/toolchain/nacl:newlib_pnacl)" ]
+  }
 }