[heap] Remove obsolete Heap::sweep_generation field.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 49bf1040b0bbc8270470717cb2ed9878042a6e07..6a0c9d58cd3d852f7a8f4228d339b3127a1e99fa 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -1337,7 +1337,7 @@ class MarkCompactMarkingVisitor
 
       // Set a number in the 0-255 range to guarantee no smi overflow.
       re->SetDataAt(JSRegExp::code_index(is_one_byte),
-                    Smi::FromInt(heap->sweep_generation() & 0xff));
+                    Smi::FromInt(heap->ms_count() & 0xff));
     } else if (code->IsSmi()) {
       int value = Smi::cast(code)->value();
       // The regexp has not been compiled yet or there was a compilation error.
@@ -1347,7 +1347,7 @@ class MarkCompactMarkingVisitor
       }
 
       // Check if we should flush now.
-      if (value == ((heap->sweep_generation() - kRegExpCodeThreshold) & 0xff)) {
+      if (value == ((heap->ms_count() - kRegExpCodeThreshold) & 0xff)) {
         re->SetDataAt(JSRegExp::code_index(is_one_byte),
                       Smi::FromInt(JSRegExp::kUninitializedValue));
         re->SetDataAt(JSRegExp::saved_code_index(is_one_byte),