[heap] Fix PagedSpace::SizeOfObjects calling from within FreeList::Allocate

[heap] Fix PagedSpace::SizeOfObjects calling from within FreeList::Allocate

Not resetting the allocation area after freeing it potentially results in
negative SizeOfObjects() because the area is accounted for as allocated while it
has already been freed.

BUG=

Review URL: https://codereview.chromium.org/1364303002

Cr-Commit-Position: refs/heads/master@{#30928}