Oilpan: Hide collection backing write barrier behind the incremental marking flag

Adding the write barrier to collection backings regressed blink_perf.parser by 16%

Bug: 818696
Change-Id: If964e2ccf39beb3010063bf785e97c60ddf007c2
Reviewed-on: https://chromium-review.googlesource.com/953443Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541733}

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

@@ -129,7 +129,9 @@ class PLATFORM_EXPORT HeapAllocator {
   static bool ExpandHashTableBacking(void*, size_t);
 
   static void BackingWriteBarrier(void* address) {
+#if BUILDFLAG(BLINK_HEAP_INCREMENTAL_MARKING)
     ThreadState::Current()->Heap().WriteBarrier(address);
+#endif
   }
 
   template <typename Return, typename Metadata>