Ninja generator: check if meminfo exists before reading it.

This CL helps cross-compile work well. Linux doesn't have to have /proc/meminfo.
For example, chroot doesn't have it.

Patch from dongseong.hwang@intel.com.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/137623016

git-svn-id: http://gyp.googlecode.com/svn/trunk@1847 78cadc50-ecff-11dd-a971-7dbc132099af