Cache the numberOfProcessors value on posix and expose it to blink

We need to cache the value, so we can request it from blink after
the sandbox was already turned on.

BUG=321060
R=jam@chromium.org,mark@chromium.org,jamesr@chromium.org

Review URL: https://codereview.chromium.org/67373006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236224 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236224