GN build: Add process_test to precompiled runtime target.

It is needed to run some I/O tests on Android devices. The buildbots were
using an existing (outdated) version, so they continue to work by accident.

R=rmacnak@google.com

Review-Url: https://codereview.chromium.org/2619723004 .

BUILD.gn

@@ -49,6 +49,7 @@ group("runtime_precompiled") {
   deps = [
     "runtime/bin:dart_bootstrap($host_toolchain)",
     "runtime/bin:dart_precompiled_runtime",
+    "runtime/bin:process_test",
     "runtime/vm:patched_sdk",
   ]
 }