Implemented file.ensureTempFile.
Implemented file.ensureTempFile.

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