Use the UNIMPLEMENTED macro everywhere.

As suggested in review.

Change-Id: Ic14c91a622b545b4800ef021dfb501fb09cbda9d
diff --git a/src/calling_convention.cc b/src/calling_convention.cc
index ec8daeb..e21b317 100644
--- a/src/calling_convention.cc
+++ b/src/calling_convention.cc
@@ -15,7 +15,7 @@
 // Managed runtime calling convention
 
 size_t ManagedRuntimeCallingConvention::FrameSize() {
-  LOG(FATAL) << "Unimplemented";
+  UNIMPLEMENTED(FATAL);
   return 0;
 }