Allow null self only in DecodeWeakGlobalDuringShutdown().

To follow up CL 169855, allow a null current thread only in
DecodeWeakGlobalDuringShutdown() as a special case rather than
DecodeWeakGlobal(). This is to prevent a bug where null is accidentally
passed to DecodeWeakGlobal().

Bug: 23897251

Change-Id: I5e7bb78ec739b8bfcf77284ed321d507737ee33e
3 files changed