https://bugs.webkit.org/show_bug.cgi?id=12216

Reviewed by Gavin Barraclough and Sam Weinig

Make the GC mark phase iterative by using an explicit mark stack.
To do this marking any single object is performed in multiple stages
  * The object is appended to the MarkStack, this sets the marked
    bit for the object using the new markDirect() function, and then
    returns
  * When the MarkStack is drain()ed the object is popped off the stack
    and markChildren(MarkStack&) is called on the object to collect
    all of its children.  drain() then repeats until the stack is empty.

Additionally I renamed a number of methods from 'mark' to 'markAggregate'
in order to make it more clear that marking of those object was not
going to result in an actual recursive mark.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@47022 268f45cc-cd09-0410-ab3c-d52691b4dbfc