Fix performance on the page cycler, which is not designed to test

GC speed.
Review URL: http://codereview.chromium.org/8804004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10148 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mark-compact.cc

@@ -3673,6 +3673,7 @@ void MarkCompactCollector::SweepSpaces() {
 #endif
   SweeperType how_to_sweep =
       FLAG_lazy_sweeping ? LAZY_CONSERVATIVE : CONSERVATIVE;
+  if (FLAG_expose_gc) how_to_sweep = CONSERVATIVE;
   if (sweep_precisely_) how_to_sweep = PRECISE;
   // Noncompacting collections simply sweep the spaces to clear the mark
   // bits and free the nonlive blocks (for old and map spaces).  We sweep