https://bugs.webkit.org/show_bug.cgi?id=122921

Reviewed by Geoffrey Garen.

Based on a patch by Mark Hahnenberg.

* API/JSBase.cpp:
(JSDisableGCTimer): Added; SPI function.
* API/JSBasePrivate.h:
* heap/BlockAllocator.cpp:
(JSC::createBlockFreeingThread): Added.
(JSC::BlockAllocator::BlockAllocator): Modified to use JSC::createBlockFreeingThread()
to conditionally create the "block freeing" thread depending on the value of
GCActivityCallback::s_shouldCreateGCTimer.
(JSC::BlockAllocator::~BlockAllocator):
* heap/BlockAllocator.h:
(JSC::BlockAllocator::deallocate):
* heap/Heap.cpp:
(JSC::Heap::didAbandon):
(JSC::Heap::collect):
(JSC::Heap::didAllocate):
* heap/HeapTimer.cpp:
(JSC::HeapTimer::timerDidFire):
* runtime/GCActivityCallback.cpp:
* runtime/GCActivityCallback.h:
(JSC::DefaultGCActivityCallback::create): Only instantiate a DefaultGCActivityCallback object
when GCActivityCallback::s_shouldCreateGCTimer is true so as to prevent allocating a HeapTimer
object (since DefaultGCActivityCallback ultimately extends HeapTimer).

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@157537 268f45cc-cd09-0410-ab3c-d52691b4dbfc