[heap] Fix Page::ShrinkToHighWaterMark.

Revision 5bc1d291 introduced a bug in the case when the filler is in
the unmapped region, which leads to CHECK failure.

Change-Id: I013e4386f6d5e9cdad22c11e1a7a9f1a088375ae
Reviewed-on: https://chromium-review.googlesource.com/823845Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50080}

src/heap/spaces.cc

@@ -895,8 +895,10 @@ size_t Page::ShrinkToHighWaterMark() {
         ClearRecordedSlots::kNo);
     heap()->memory_allocator()->PartialFreeMemory(
         this, address() + size() - unused, unused, area_end() - unused);
-    CHECK(filler->IsFiller());
-    CHECK_EQ(filler->address() + filler->Size(), area_end());
+    if (filler->address() != area_end()) {
+      CHECK(filler->IsFiller());
+      CHECK_EQ(filler->address() + filler->Size(), area_end());
+    }
   }
   return unused;
 }