Use the UNIMPLEMENTED macro everywhere.

As suggested in review.

Change-Id: Ic14c91a622b545b4800ef021dfb501fb09cbda9d
diff --git a/src/mark_sweep.cc b/src/mark_sweep.cc
index aefd502..ffc3977 100644
--- a/src/mark_sweep.cc
+++ b/src/mark_sweep.cc
@@ -70,7 +70,7 @@
 
 // Marks all objects in the root set.
 void MarkSweep::MarkRoots() {
-  LOG(FATAL) << "Unimplemented";
+  UNIMPLEMENTED(FATAL);
 }
 
 void MarkSweep::ScanBitmapCallback(Object* obj, void* finger, void* arg) {
@@ -96,11 +96,11 @@
 }
 
 void MarkSweep::ReMarkRoots() {
-  LOG(FATAL) << "Unimplemented";
+  UNIMPLEMENTED(FATAL);
 }
 
 void MarkSweep::SweepSystemWeaks() {
-  LOG(FATAL) << "Unimplemented";
+  UNIMPLEMENTED(FATAL);
 }
 
 void MarkSweep::SweepCallback(size_t num_ptrs, void **ptrs, void *arg) {
@@ -461,7 +461,7 @@
     // JValue unused;
     // Object *reference = *cleared;
     // TODO: dvmCallMethod(self, meth, NULL, &unused, reference);
-    LOG(FATAL) << "Unimplemented";
+    UNIMPLEMENTED(FATAL);
     *cleared = NULL;
   }
 }