Implemented ProxyObject.checkInvariants.
Implemented ProxyObject.checkInvariants.

mbrukman/gcsfuse,jacobsa/ratelimit,BanzaiMan/gcsfuse,kahing/gcsfuse,jacobsa/timeutil,GoogleCloudPlatform/gcsfuse,BanzaiMan/gcsfuse,GoogleCloudPlatform/gcsfuse,GoogleCloudPlatform/gcsfuse