Implement committed physical memory stats for Linux.

src/platform-freebsd.cc

Index: src/platform-freebsd.cc
diff --git a/src/platform-freebsd.cc b/src/platform-freebsd.cc
index 511759c485e245891dd41a2fa792128e91915f91..7d41f37083fdd429c946c8823ce71504a161bb7a 100644
--- a/src/platform-freebsd.cc
+++ b/src/platform-freebsd.cc
@@ -456,6 +456,13 @@ bool VirtualMemory::ReleaseRegion(void* base, size_t size) {
 }
 
 
+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:
   pthread_t thread_;  // Thread handle for pthread.