Require mutator lock for DeleteLocalRef

There was a race condition where suspended threads could call
DeleteLocalRef while the GC was marking their roots. This could
cause the GC to attempt to mark a null object.

(cherry picked from commit dd06afecd67fc6a60856436ab150a4d1b9a8ad29)

Bug: 22119403

Change-Id: I962c717bb87b2acb2a4710a2d7ab16793e031401
1 file changed