Mark inspector/tracing.html as flaky on Mac

BUG=264998
TBR=pfeldman

Review URL: https://codereview.chromium.org/20822004

git-svn-id: svn://svn.chromium.org/blink/trunk@155035 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Removing a test configuration from a NeedsRebaseline line should not change to Skip.

Before this patch, to_string wouldn't serialize anything that wasn't
in the EXPECTATION_ORDER tuple, resulting in empty expectations lists,
which in turn get serialized as [ Skip ].

R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/20708004

git-svn-id: svn://svn.chromium.org/blink/trunk@155036 bbb929c8-8fbe-4397-9dbb-9b2b20218538