Implement committed physical memory stats for Linux.

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index f50e7130be699682b21a445b8fee6e954d2fae88..5e3bdc7b8894a147324e5ab63f71b1b8ff54b457 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -429,6 +429,14 @@ class VirtualMemory {
   // and the same size it was reserved with.
   static bool ReleaseRegion(void* base, size_t size);
 
+  // Returns the size of committed memory which is currently resident
+  // in the physical memory for the region specified with base and size
+  // arguments.
+  // On success stores the committed physical memory size at the location
+  // pointed by the last argument and returns true. Returns false on failure.
+  static bool CommittedPhysicalSizeInRegion(
+      void* base, size_t size, size_t* physical);
+
  private:
   void* address_;  // Start address of the virtual memory.
   size_t size_;  // Size of the virtual memory.