Fix os.path.exists on symlink.

This would fail on the os.symlink below if the symlink was broken.

Change-Id: If055cdeb0353bc39440717e09bed4666678b121a
diff --git a/tools/heap_profile b/tools/heap_profile
index f923c7f..7ed9d42 100755
--- a/tools/heap_profile
+++ b/tools/heap_profile
@@ -300,7 +300,7 @@
 
   symlink_path = os.path.join(os.path.dirname(profile_path),
                                         "heap_profile-latest")
-  if os.path.exists(symlink_path):
+  if os.path.lexists(symlink_path):
     os.unlink(symlink_path)
   os.symlink(profile_path, symlink_path)