Some MallocBench refinements

Some MallocBench refinements
https://bugs.webkit.org/show_bug.cgi?id=136750

Reviewed by Sam Weinig.

* MallocBench/MallocBench/Interpreter.cpp:
(Interpreter::run): Allow for null entries in the object list so that
we can test in modes that exclude large or small allocations.

* MallocBench/MallocBench/churn.cpp:
(benchmark_churn):
* MallocBench/MallocBench/flickr.cpp:
(benchmark_flickr):
* MallocBench/MallocBench/fragment.cpp:
(benchmark_fragment_iterate):
* MallocBench/MallocBench/list.cpp:
(benchmark_list_allocate):
* MallocBench/MallocBench/reddit.cpp:
(benchmark_reddit): Updated test runtimes to weight them more equally,
for the sake of arithmetic mean.

* MallocBench/MallocBench/stress.cpp:
(Object::Object):
(allocate):
(deallocate):
(benchmark_stress): Verify the contents of memory as we go. Also,
force scavenging each time through the loop to test the scavenging path.

* MallocBench/MallocBench/theverge.cpp:
(benchmark_theverge):
* MallocBench/MallocBench/tree.cpp:
(benchmark_tree_churn): Re-weighted, as above.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@173529 268f45cc-cd09-0410-ab3c-d52691b4dbfc