Implement committed physical memory stats for Linux.

src/platform-nullos.cc

Index: src/platform-nullos.cc
diff --git a/src/platform-nullos.cc b/src/platform-nullos.cc
index 679ef8e89e308da2efd53f1e1e44e02b39dab2b9..0b2929d7437845d2d59f347e9afafb0bbb068e0b 100644
--- a/src/platform-nullos.cc
+++ b/src/platform-nullos.cc
@@ -335,6 +335,13 @@ bool VirtualMemory::Guard(void* address) {
 }
 
 
+bool VirtualMemory::CommittedPhysicalSizeInRegion(
+    void* base, size_t size, size_t* physical) {
+  // TODO(alph): implement for the platform.
+  return false;
+}
+
+
 class Thread::PlatformData : public Malloced {
  public:
   PlatformData() {