[vm, gc] Shorten extent of the freelist lock during scavenge.

Helps break a cycle for https://dart-review.googlesource.com/c/sdk/+/435241

TEST=ci
Change-Id: I98994a3d2f5e11f0f9bb20e38d3c2b9fa9e48d08
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/436521
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/vm/heap/scavenger.cc

@@ -320,6 +320,7 @@ class ScavengerVisitor : public ObjectPointerVisitor,
   }
 
   void ProcessWeak() {
+    page_space_->ReleaseLock(freelist_);
     if (!scavenger_->abort_) {
       ASSERT(!HasWork());
 
@@ -334,7 +335,6 @@ class ScavengerVisitor : public ObjectPointerVisitor,
       MournFinalizerEntries();
       scavenger_->IterateWeak();
     }
-    page_space_->ReleaseLock(freelist_);
     thread_ = nullptr;
   }