Merge branch 'master' into stable-6.5

* master:
  If tryLock fails to get the lock another gc has it
  Fix GcConcurrentTest#testInterruptGc
  Don't swallow IOException in GC.PidLock#lock
  Check if FileLock is valid before using or releasing it

Change-Id: I0a599ec9241dd3b9b9a61122b2eda6bd647bcc22