Verify live bytes only if enabled on command line

Change-Id: If46c25c098565bcb43f5475783591190f6d80b56
Reviewed-on: https://chromium-review.googlesource.com/1096040Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#53655}

src/heap/mark-compact.cc

@@ -832,9 +832,11 @@ void MarkCompactCollector::VerifyMarking() {
   }
 #endif
 #ifdef VERIFY_HEAP
+  if (FLAG_verify_heap) {
     heap()->old_space()->VerifyLiveBytes();
     heap()->map_space()->VerifyLiveBytes();
     heap()->code_space()->VerifyLiveBytes();
+  }
 #endif
 }