Implement committed physical memory stats for Linux.

src/platform-openbsd.cc

Index: src/platform-openbsd.cc
diff --git a/src/platform-openbsd.cc b/src/platform-openbsd.cc
index 408d4dc0f8487e08a8a4277ad8139f4910b6d4de..558361dbad15afc795113a2027bd751535efb2df 100644
--- a/src/platform-openbsd.cc
+++ b/src/platform-openbsd.cc
@@ -504,6 +504,13 @@ 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:
   PlatformData() : thread_(kNoThread) {}