[oilpan] Fix object start bitmap verification

Verification requires iterability of pages implying that we cannot have
a linear allocation area.

The area was however potentially set up during compaction.

Bug: chromium:757440
Change-Id: I9b12ecac032893aaa6d6f022149cec0a6130ebf9
Reviewed-on: https://chromium-review.googlesource.com/997672Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548421}

third_party/WebKit/Source/platform/heap/HeapPage.cpp

@@ -210,7 +210,7 @@ void BaseArena::MakeConsistentForMutator() {
   }
   DCHECK(SweepingCompleted());
 
-  Verify();
+  VerifyObjectStartBitmap();
 }
 
 size_t BaseArena::ObjectPayloadSizeForTesting() {
@@ -232,7 +232,7 @@ void BaseArena::PrepareForSweep() {
   ClearFreeLists();
 
   // Verification depends on the allocation point being cleared.
-  Verify();
+  VerifyObjectStartBitmap();
 
   for (BasePage* page = first_page_; page; page = page->Next()) {
     page->MarkAsUnswept();
@@ -584,11 +584,14 @@ void NormalPageArena::SweepAndCompact() {
   heap.Compaction()->FinishedArenaCompaction(this, freed_page_count,
                                              freed_size);
 
-  Verify();
+  VerifyObjectStartBitmap();
 }
 
-void NormalPageArena::Verify() {
+void NormalPageArena::VerifyObjectStartBitmap() {
 #if DCHECK_IS_ON()
+  // Verifying object start bitmap requires iterability of pages. As compaction
+  // may set up a new we have to reset here.
+  SetAllocationPoint(nullptr, 0);
   for (NormalPage* page = static_cast<NormalPage*>(first_page_); page;
        page = static_cast<NormalPage*>(page->Next()))
     page->VerifyObjectStartBitmapIsConsistentWithPayload();

third_party/WebKit/Source/platform/heap/HeapPage.h

@@ -774,7 +774,7 @@ class PLATFORM_EXPORT BaseArena {
 
   bool WillObjectBeLazilySwept(BasePage*, void*) const;
 
-  virtual void Verify(){};
+  virtual void VerifyObjectStartBitmap(){};
   virtual void VerifyMarking(){};
 
  protected:
@@ -836,7 +836,7 @@ class PLATFORM_EXPORT NormalPageArena final : public BaseArena {
 
   void SweepAndCompact();
 
-  void Verify() override;
+  void VerifyObjectStartBitmap() override;
   void VerifyMarking() override;
 
   Address CurrentAllocationPoint() const { return current_allocation_point_; }