Wait on the semaphore for all finished unmapper tasks.

The task count was reset after handling the first task, so the wait
loop would end after one iteration. As a result unmapping tasks might
have been left running even after waiting for the tasks to be
completed.

Bug: v8:7412
Change-Id: I869848477946589d0370ab038d135393e816a7c0
Reviewed-on: https://chromium-review.googlesource.com/903166Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51127}

src/heap/spaces.cc

@@ -354,8 +354,8 @@ void MemoryAllocator::Unmapper::WaitUntilCompleted() {
         CancelableTaskManager::kTaskAborted) {
       pending_unmapping_tasks_semaphore_.Wait();
     }
-    concurrent_unmapping_tasks_active_ = 0;
   }
+  concurrent_unmapping_tasks_active_ = 0;
 }
 
 template <MemoryAllocator::Unmapper::FreeMode mode>