[heap] Disable moving object starts aka left trimming

BUG=

Review URL: https://codereview.chromium.org/1301963002

Cr-Commit-Position: refs/heads/master@{#30303}

src/flag-definitions.h

@@ -674,6 +674,7 @@ DEFINE_IMPLICATION(trace_detached_contexts, track_detached_contexts)
 #ifdef VERIFY_HEAP
 DEFINE_BOOL(verify_heap, false, "verify heap pointers before and after GC")
 #endif
+DEFINE_BOOL(move_object_start, false, "enable moving of object starts")
 
 // counters.cc
 DEFINE_INT(histogram_interval, 600000,

src/heap/heap.cc

@@ -3608,6 +3608,8 @@ void Heap::CreateFillerObjectAt(Address addr, int size) {
 
 
 bool Heap::CanMoveObjectStart(HeapObject* object) {
+  if (!FLAG_move_object_start) return false;
+
   Address address = object->address();
 
   if (lo_space()->Contains(object)) return false;