[heap] add assertions to ensure step size remains positive

These assertions would have caught the bug closer to the source.

BUG=v8:7313

Change-Id: I19b75c3fc7bff8216dda370114ff00326f128116
Reviewed-on: https://chromium-review.googlesource.com/887175Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#50910}

src/heap/heap.cc

@@ -6688,6 +6688,7 @@ void AllocationObserver::AllocationStep(int bytes_allocated,
     step_size_ = GetNextStepSize();
     bytes_to_next_step_ = step_size_;
   }
+  DCHECK_GE(bytes_to_next_step_, 0);
 }
 
 namespace {

src/heap/spaces.cc

@@ -1371,7 +1371,7 @@ intptr_t Space::GetNextInlineAllocationStepSize() {
     next_step = next_step ? Min(next_step, observer->bytes_to_next_step())
                           : observer->bytes_to_next_step();
   }
-  DCHECK(allocation_observers_.size() == 0 || next_step != 0);
+  DCHECK(allocation_observers_.size() == 0 || next_step > 0);
   return next_step;
 }