[heap] make {New,Paged}Space::PauseAllocationObserver more consistent

Bring the two functions closer together in implementation. Avoid
unnecessary assignment to top_on_previous_step_.

Bug: 
Change-Id: I0b6881b02ab08a48bc2051e6786e22a7b25a4948
Reviewed-on: https://chromium-review.googlesource.com/815196Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#49944}

src/heap/spaces.cc

@@ -2176,9 +2176,9 @@ void SpaceWithLinearArea::RemoveAllocationObserver(
 
 void NewSpace::PauseAllocationObservers() {
   // Do a step to account for memory allocated so far.
-  InlineAllocationStep(top(), top(), nullptr, 0);
+  InlineAllocationStep(top(), nullptr, nullptr, 0);
   Space::PauseAllocationObservers();
-  top_on_previous_step_ = 0;
+  DCHECK_NULL(top_on_previous_step_);
   UpdateInlineAllocationLimit(0);
 }
 
@@ -2188,7 +2188,7 @@ void PagedSpace::PauseAllocationObservers() {
   // from NewSpace version.
   InlineAllocationStep(top(), nullptr, nullptr, 0);
   Space::PauseAllocationObservers();
-  top_on_previous_step_ = 0;
+  DCHECK_NULL(top_on_previous_step_);
 }
 
 void SpaceWithLinearArea::ResumeAllocationObservers() {