Drop DECLARE_GC_INFO from PerformanceResourceTiming

We shouldn't add DECLARE_GC_INFO to a subclass of garbage-collected class.
Otherwise, we will hit an assertion about type marker:
https://codereview.chromium.org/170143002/#msg22

This CL fixes a crash of test_resource_dedicated_worker.html in oilpan build bots.

R=ager@chromium.org

Review URL: https://codereview.chromium.org/173673002

git-svn-id: svn://svn.chromium.org/blink/trunk@167507 bbb929c8-8fbe-4397-9dbb-9b2b20218538