MallocBench should scavenge explicitly instead of waiting

MallocBench should scavenge explicitly instead of waiting
https://bugs.webkit.org/show_bug.cgi?id=131661

Reviewed by Andreas Kling.

PerformanceTests:
* MallocBench/MallocBench.xcodeproj/project.pbxproj: Don't build mbmalloc
by default because it will overwrite any other mbmalloc you're working
with in the WebKitBuild directory.

* MallocBench/MallocBench/Benchmark.cpp:
(Benchmark::run): Scavenge explicitly instead of waiting. This is faster,
and it's the only way to get FastMalloc to scavenge. (That's a bug in
FastMalloc, but we don't want it to interfere with broader testing.)

* MallocBench/MallocBench/mbmalloc.cpp:
* MallocBench/MallocBench/mbmalloc.h: Added a scavenge implementation
for system malloc.

Source/bmalloc:
Added explicit scavenge support to bmalloc. This isn't a memory win,
since bmalloc's per-thread cache is so small. But it makes testing
simpler.

* bmalloc/Allocator.cpp:
(bmalloc::Allocator::~Allocator):
(bmalloc::Allocator::scavenge):
* bmalloc/Allocator.h:
* bmalloc/Cache.cpp:
(bmalloc::Cache::operator new):
(bmalloc::Cache::operator delete):
(bmalloc::Cache::Cache):
(bmalloc::Cache::scavenge):
* bmalloc/Cache.h:
* bmalloc/Deallocator.cpp:
(bmalloc::Deallocator::~Deallocator):
(bmalloc::Deallocator::scavenge):
* bmalloc/Deallocator.h: Factored existing scavenging code into helper
functions, for reuse.

* bmalloc/Heap.cpp:
(bmalloc::sleep):
(bmalloc::Heap::concurrentScavenge):
(bmalloc::Heap::scavenge):
(bmalloc::Heap::scavengeSmallPages):
(bmalloc::Heap::scavengeMediumPages):
(bmalloc::Heap::scavengeLargeRanges):
* bmalloc/Heap.h: Made scavenge sleep duration a parameter. Forced
scavenging -- in response to a benchmark or a low memory warning --
wants to complete as soon as possible, so its sleep duration is 0.

* bmalloc/bmalloc.h:
(bmalloc::api::scavenge):
* bmalloc/mbmalloc.cpp: Exported the scavenge API for MallocBench's use.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@167292 268f45cc-cd09-0410-ab3c-d52691b4dbfc