Restrict ParallelJobs to at most as many threads as there are cores.
Restrict ParallelJobs to at most as many threads as there are cores.

BUG=336795

Review URL: https://codereview.chromium.org/146743002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@165775 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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