Add mojo_system_unittest back to valgrind bots, which was removed by accident...

Add mojo_system_unittest back to valgrind bots, which was removed by accident in https://codereview.chromium.org/681493003

R=jamesr@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/683493004

Cr-Commit-Position: refs/heads/master@{#301468}

tools/valgrind/chrome_tests.py

@@ -690,6 +690,7 @@ class ChromeTests:
     "message_center": TestMessageCenter,
     "message_center_unittests" : TestMessageCenter,
     "mojo_common": TestMojoCommon,
+    "mojo_system": TestMojoSystem,
     "mojo_public_system": TestMojoPublicSystem,
     "mojo_public_utility": TestMojoPublicUtility,
     "mojo_public_bindings": TestMojoPublicBindings,