[heap] Add the free remainder of a black page to the free list.

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index 89e0c9bc376c8ee25ccf1380ccf8afe733d50d7e..8723b77411e16170a6ee66f031b26df65e77831b 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -665,6 +665,8 @@ class MemoryChunk {
   // Approximate amount of physical memory committed for this chunk.
   size_t CommittedPhysicalMemory() { return high_water_mark_.Value(); }
 
+  Address HighWaterMark() { return address() + high_water_mark_.Value(); }
+
   int progress_bar() {
     DCHECK(IsFlagSet(HAS_PROGRESS_BAR));
     return progress_bar_;