Return configured processors in NumberOfProcessors() on posix platforms

On Linux/Mac/FreeBSD platform, NumberOfProcessors() was returning the number of
online processors. Change it to return the number of configured processors
(incl. offline processors).

1. Most callers of this API expect it to return totally available cpu cores,
   mainly for threading optimization.

2. The number of online processors is changed within seconds on some embedded
   platforms. It is wrong to pass the cached value of the number of currently
   online processors to Blink sandbox.

BUG=404364
TEST=Tested on nyan (it may have 1~4 cpu cores online)

Review URL: https://codereview.chromium.org/454053002

git-svn-id: http://src.chromium.org/svn/trunk/src/base@290209 4ff67af0-8c30-449e-8e8b-ad334ec8d88c