[webkitpy] Use $NUMBER_OF_PROCESSORS in Executive.cpu_count()
https://bugs.webkit.org/show_bug.cgi?id=114506

Patch by Manuel Rego Casasnovas <rego@igalia.com> on 2013-04-15
Reviewed by Philippe Normand.

* Scripts/webkitpy/common/system/executive.py:
(Executive.cpu_count): Modify function in order to use the environment
variable $NUMBER_OF_PROCESSORS if it is defined and it is an integer
greater than 0.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148429 268f45cc-cd09-0410-ab3c-d52691b4dbfc