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

diff --git a/base/process_util.h b/base/process_util.h
index 3d9b7d1..5f88c62 100644
--- a/base/process_util.h
+++ b/base/process_util.h

@@ -653,7 +653,14 @@
   DISALLOW_COPY_AND_ASSIGN(ProcessMetrics);
 };
 
-// Returns the memory commited by the system in KBytes.
+#if defined(OS_LINUX)
+// Retrieves data from /proc/meminfo about system-wide memory consumption.
+// Values are in KB. Returns true on success.
+BASE_API bool GetSystemMemoryInfo(int* total_kb, int* free_kb, int* buffers_kb,
+                                  int* cache_kb, int* shmem_kb);
+#endif
+
+// Returns the memory committed by the system in KBytes.
 // Returns 0 if it can't compute the commit charge.
 BASE_API size_t GetSystemCommitCharge();