[heap] Remove dead last top marker

Bug: 
Change-Id: I1bb6c6e3853317851544ca9f4eba1df76a147bfb
Reviewed-on: https://chromium-review.googlesource.com/707317Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48416}

src/heap/heap.cc

@@ -935,9 +935,6 @@ void Heap::GarbageCollectionEpilogue() {
   ReportStatisticsAfterGC();
 #endif  // DEBUG
 
-  // Remember the last top pointer so that we can later find out
-  // whether we allocated in new space since the last GC.
-  new_space_top_after_last_gc_ = new_space()->top();
   last_gc_time_ = MonotonicallyIncreasingTimeInMs();
 
   {
@@ -5431,7 +5428,6 @@ bool Heap::SetUp() {
   if (!new_space_->SetUp(initial_semispace_size_, max_semi_space_size_)) {
     return false;
   }
-  new_space_top_after_last_gc_ = new_space()->top();
 
   space_[OLD_SPACE] = old_space_ =
       new OldSpace(this, OLD_SPACE, NOT_EXECUTABLE);

src/heap/heap.h

@@ -2258,7 +2258,6 @@ class Heap {
   Space* space_[LAST_SPACE + 1];
   HeapState gc_state_;
   int gc_post_processing_depth_;
-  Address new_space_top_after_last_gc_;
 
   // Returns the amount of external memory registered since last global gc.
   uint64_t PromotedExternalMemorySize();