Oilpan: add temporary plugin ignore exemption for DataRef<T>::RefPtr<T>.

Until the clang Blink GC plugin has been updated to include r359074,
the GC plugin will erroneously complain if DataRef<T> is used with a
RefCountedGarbageCollected<T> class type. Add a GC_PLUGIN_IGNORE()
to temporarily prevent such false errors from being reported.

R=
BUG=553613

Review URL: https://codereview.chromium.org/1440553002

Cr-Commit-Position: refs/heads/master@{#359094}
1 file changed