CrOS - Add memory consumption status bar widget behind flag

Pass --memory-widget on CrOS to get a real-time display, updated every 5 seconds, of the system's free memory.  Tooltip and menu itself detail the data from /proc/meminfo.  Also made base::GetSystemMemoryInfo() available on Linux systems to provide detailed data.

BUG=chromium-os:18446
TEST=manual

Review URL: http://codereview.chromium.org/7518010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@95089 0039d316-1c4b-4281-b951-d872f2087c98

• base/process_util.h[diff]
• base/process_util_linux.cc[diff]
• chrome/browser/chromeos/status/memory_menu_button.cc[Added - diff]
• chrome/browser/chromeos/status/memory_menu_button.h[Added - diff]
• chrome/browser/chromeos/status/status_area_view.cc[diff]
• chrome/browser/chromeos/status/status_area_view.h[diff]
• chrome/chrome_browser.gypi[diff]
• chrome/common/chrome_switches.cc[diff]
• chrome/common/chrome_switches.h[diff]