Fix flaky test-inobject-slack-tracking cctest.

The test does not take into account that Turbofan can force completion
of in-object slack tracking.

Bug: 
Change-Id: Ic7404e8395858acaf707e65ca4b17e6b7f776594
Reviewed-on: https://chromium-review.googlesource.com/723464Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48687}

test/cctest/test-inobject-slack-tracking.cc

@@ -596,6 +596,10 @@ static void TestClassHierarchy(const std::vector<int>& hierarchy_desc, int n) {
       Handle<JSObject> tmp = RunI<JSObject>(new_script);
       CHECK_EQ(initial_map->IsInobjectSlackTrackingInProgress(),
                IsObjectShrinkable(*tmp));
+      if (!initial_map->IsInobjectSlackTrackingInProgress()) {
+        // Turbofan can force completion of in-object slack tracking.
+        break;
+      }
       CHECK_EQ(Map::kSlackTrackingCounterStart - i - 1,
                initial_map->construction_counter());
     }