Revert "Increase the target new space size to the max new space size"

TBR=hpayer@chromium.org
BUG=v8:3626
LOG=n

Review URL: https://codereview.chromium.org/724543003

Cr-Commit-Position: refs/heads/master@{#25314}

src/heap/heap.cc

@@ -5040,10 +5040,9 @@ bool Heap::ConfigureHeap(int max_semi_space_size, int max_old_space_size,
     } else {
       target_semispace_size_ = target_semispace_size;
     }
-  } else {
-    target_semispace_size_ = max_semi_space_size_;
   }
 
+  target_semispace_size_ = Max(initial_semispace_size_, target_semispace_size_);
 
   // The old generation is paged and needs at least one page for each space.
   int paged_space_count = LAST_PAGED_SPACE - FIRST_PAGED_SPACE + 1;