De-duplicate code by adding size-independent iteration.

runtime/vm/pages.h

Index: runtime/vm/pages.h
===================================================================
--- runtime/vm/pages.h	(revision 35671)
+++ runtime/vm/pages.h	(working copy)
@@ -296,6 +296,10 @@
   HeapPage* AllocateLargePage(intptr_t size, HeapPage::PageType type);
   void FreeLargePage(HeapPage* page, HeapPage* previous_page);
   void FreePages(HeapPage* pages);
+  HeapPage* NextPageAnySize(HeapPage* page) const {
+    ASSERT(pages_tail_ == NULL || pages_tail_->next() == NULL);
+    return page == pages_tail_ ? large_pages_ : page->next();
+  }
 
   static intptr_t LargePageSizeInWordsFor(intptr_t size);