bmalloc scavenger should know what page classes are allocating

bmalloc scavenger should know what page classes are allocating
https://bugs.webkit.org/show_bug.cgi?id=171384

Reviewed by Geoffrey Garen.

This change replaces m_isAllocatingPages with a per page class flag to track which page
classes are currently allocating.  When scavenging, we skip page classes that are actively
allocating and come back to them on a subsequent pass.  This reduces the amount of time it
takes for scavenger to free up pages as well as the total time it takes to handle all
page classes.

* bmalloc/Heap.cpp:
(bmalloc::Heap::Heap):
(bmalloc::Heap::concurrentScavenge):
(bmalloc::Heap::scavenge):
(bmalloc::Heap::scavengeSmallPages):
(bmalloc::Heap::scavengeLargeObjects):
(bmalloc::Heap::allocateSmallPage):
(bmalloc::Heap::splitAndAllocate):
(bmalloc::Heap::deallocateLarge):
* bmalloc/Heap.h:
(bmalloc::Heap::takeRequestedScavengerThreadQOSClass): Deleted.
* bmalloc/VMHeap.h:
(bmalloc::VMHeap::deallocateSmallPage):
* bmalloc/bmalloc.h:
(bmalloc::api::scavenge):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@215909 268f45cc-cd09-0410-ab3c-d52691b4dbfc