Review Request 23627: Refactored Docker::Container::pid() to return an Option.

[Benjamin Hindman]

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https://reviews.apache.org/r/23627/
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Review request for mesos, Ian Downes, Jie Yu, and Timothy Chen.


Bugs: MESOS-1524
    https://issues.apache.org/jira/browse/MESOS-1524


Repository: mesos-git


Description
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See summary.


Diffs
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  src/docker/docker.hpp PRE-CREATION 
  src/docker/docker.cpp PRE-CREATION 
  src/slave/containerizer/docker.cpp PRE-CREATION 

Diff: https://reviews.apache.org/r/23627/diff/


Testing
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make check


Thanks,

Benjamin Hindman