debug: turn off sparse bitmap pointer alignment.

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

Index: third_party/WebKit/Source/platform/heap/HeapPage.cpp
diff --git a/third_party/WebKit/Source/platform/heap/HeapPage.cpp b/third_party/WebKit/Source/platform/heap/HeapPage.cpp
index 8dc5d2d0155f2c76ae5d63252c4a9b1b2475f910..7078cfc0017398d52b37f261edefb2c40fc0c91c 100644
--- a/third_party/WebKit/Source/platform/heap/HeapPage.cpp
+++ b/third_party/WebKit/Source/platform/heap/HeapPage.cpp
@@ -563,6 +563,13 @@ void NormalPageArena::sweepAndCompact() {
     freedPageCount++;
     BasePage* nextPage;
     availablePages->unlink(&nextPage);
+    // Clear out the page before adding it to the free page pool, which
+    // decommits it. Recommitting the page must find a zeroed page later.
+    // We cannot assume that the OS will hand back a zeroed page across
+    // its "decommit" operation.
+    DCHECK(!availablePages->isLargeObjectPage());
+    NormalPage* unusedPage = reinterpret_cast<NormalPage*>(availablePages);
+    memset(unusedPage->payload(), 0, unusedPage->payloadSize());
     availablePages->removeFromHeap();
     availablePages = static_cast<NormalPage*>(nextPage);
   }
@@ -1504,8 +1511,7 @@ void NormalPage::sweepAndCompact(CompactionContext& context) {
 
 #if ENABLE(ASSERT) || defined(LEAK_SANITIZER) || defined(ADDRESS_SANITIZER) || \
     defined(MEMORY_SANITIZER)
-  // Zap the page, which is now available and will either be compacted into
-  // or freed.
+  // Zap the unused portion, until it is either compacted into or freed.
   if (currentPage != this) {
     FreeList::zapFreedMemory(payload(), payloadSize());
   } else {