Fixing a subtle bug where in some cases the post cancellation work wasn't...

Fixing a subtle bug where in some cases the post cancellation work wasn't being done correctly. This is the scenario in which FunctionBufferingResource::Cancel() got called while buffering was being done, but then the buffer filled up in which case FillBuffer() wasn't ever called and the Cancel() method would get stuck waiting on a notification from the condition variable leading to timeouts. This CL fixes this by making sure FillBuffer() got called one last time in this case.

Tested by running contrib/data/python/kernel_tests:prefetching_ops_test 500 times and ran contrib/distribute/python:values_test 500 times with no timeouts.

PiperOrigin-RevId: 191007895