Implement committed physical memory stats for Linux.

Implement committed physical memory stats for Linux.

The patch introduces CommittedPhysicalMemory function to the Heap class
that reports committed *physical* memory acquired from the OS.
It is important because some OSes may postpone actual commitment on e.g.
first access to the previously committed region.
So reporting just plain committed size led to various weird artifacts
like DevTools showing V8 allocated memory higher than the whole process
private size.

BUG=v8:2191

Review URL: https://codereview.chromium.org/10961042
Patch from Alexei Filippov <alph@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12625 ce2b1a6d-e550-0410-aec6-3dcde31c8c00