Implement committed physical memory stats for Linux.

src/platform-macos.cc

Index: src/platform-macos.cc
diff --git a/src/platform-macos.cc b/src/platform-macos.cc
index a216f6e4cacd2561cb983550ed64d6974dbd095f..9b14d2f0c6dc225c74784fc6ca69fb23f24c389c 100644
--- a/src/platform-macos.cc
+++ b/src/platform-macos.cc
@@ -471,6 +471,13 @@ bool VirtualMemory::ReleaseRegion(void* address, 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) {}