[heap] Lower limit for maximum number of unmapper tasks

Bug: chromium:791043
Change-Id: Ica59510f9ee611d62bc65775977df7c7f99fb8ca
Reviewed-on: https://chromium-review.googlesource.com/807929Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49876}

src/heap/spaces.h

@@ -1229,7 +1229,7 @@ class V8_EXPORT_PRIVATE MemoryAllocator {
 
    private:
     static const int kReservedQueueingSlots = 64;
-    static const int kMaxUnmapperTasks = 24;
+    static const int kMaxUnmapperTasks = 4;
 
     enum ChunkQueueType {
       kRegular,     // Pages of kPageSize that do not live in a CodeRange and