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..cdda36d900e0511c47da811c4ba30c60abd543f8 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -429,6 +429,11 @@ class VirtualMemory {
   // and the same size it was reserved with.
   static bool ReleaseRegion(void* base, size_t size);
 
+  // Returns true if OS performs lazy commits, i.e. the memory allocation call
+  // defers actual physical memory allocation till the first memory access.
+  // Otherwise returns false.
+  static bool HasLazyCommits();
+
  private:
   void* address_;  // Start address of the virtual memory.
   size_t size_;  // Size of the virtual memory.