A per-heap RegionTree needs no lock.

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

Index: third_party/WebKit/Source/platform/heap/PageMemory.h
diff --git a/third_party/WebKit/Source/platform/heap/PageMemory.h b/third_party/WebKit/Source/platform/heap/PageMemory.h
index 6e1e87a24d91de89b067376e31b5d8f808f8808b..894c9bc1586a8b8c89f3612ce20b9f044bac1c56 100644
--- a/third_party/WebKit/Source/platform/heap/PageMemory.h
+++ b/third_party/WebKit/Source/platform/heap/PageMemory.h
@@ -11,11 +11,6 @@
 #include "wtf/Compiler.h"
 #include "wtf/allocator/Partitions.h"
 
-#if OS(POSIX)
-#include <sys/mman.h>
-#include <unistd.h>
-#endif
-
 namespace blink {
 
 class RegionTree;
@@ -122,7 +117,6 @@ class RegionTree {
   PageMemoryRegion* lookup(Address);
 
  private:
-  Mutex m_mutex;
   RegionTreeNode* m_root;
 };