[heap] Do not allow left trimming on new space pages that need to be swept.

BUG=chromium:630969,chromium:632334
LOG=n

Review-Url: https://codereview.chromium.org/2189033002
Cr-Commit-Position: refs/heads/master@{#38137}

src/heap/heap.cc

@@ -3083,14 +3083,8 @@ bool Heap::CanMoveObjectStart(HeapObject* object) {
 
   if (lo_space()->Contains(object)) return false;
 
-  Page* page = Page::FromAddress(address);
-  // We can move the object start if:
-  // (1) the object is not in old space,
-  // (2) the page of the object was already swept,
-  // (3) the page was already concurrently swept. This case is an optimization
-  // for concurrent sweeping. The WasSwept predicate for concurrently swept
-  // pages is set after sweeping all pages.
-  return !InOldSpace(object) || page->SweepingDone();
+  // We can move the object start if the page was already swept.
+  return Page::FromAddress(address)->SweepingDone();
 }