[heap] Remove unflattened_string_length

BUG=

Review URL: https://codereview.chromium.org/1303443005

Cr-Commit-Position: refs/heads/master@{#30267}

src/heap/heap.cc

@@ -88,7 +88,6 @@ Heap::Heap()
       ms_count_(0),
       gc_count_(0),
       remembered_unmapped_pages_index_(0),
-      unflattened_strings_length_(0),
 #ifdef DEBUG
       allocation_timeout_(0),
 #endif  // DEBUG
@@ -401,7 +400,6 @@ void Heap::GarbageCollectionPrologue() {
   {
     AllowHeapAllocation for_the_first_part_of_prologue;
     gc_count_++;
-    unflattened_strings_length_ = 0;
 
     if (FLAG_flush_code) {
       mark_compact_collector()->EnableCodeFlushing(true);

src/heap/heap.h

@@ -1743,9 +1743,6 @@ class Heap {
   int remembered_unmapped_pages_index_;
   Address remembered_unmapped_pages_[kRememberedUnmappedPages];
 
-  // Total length of the strings we failed to flatten since the last GC.
-  int unflattened_strings_length_;
-
 #define ROOT_ACCESSOR(type, name, camel_name) \
   inline void set_##name(type* value);
   ROOT_LIST(ROOT_ACCESSOR)