reland 2: Switch back to field trial.

base/memory/shared_memory_handle.h

Index: base/memory/shared_memory_handle.h
diff --git a/base/memory/shared_memory_handle.h b/base/memory/shared_memory_handle.h
index b3dfc8fa380e220a62e8384dfb09000e975a1386..49398a4a9a96cf066e8ff830b703c89c16cb9626 100644
--- a/base/memory/shared_memory_handle.h
+++ b/base/memory/shared_memory_handle.h
@@ -75,12 +75,15 @@ class BASE_EXPORT SharedMemoryHandle {
 #else
 class BASE_EXPORT SharedMemoryHandle {
  public:
+  // The values of these enums must not change, as they are used by the
+  // histogram OSX.SharedMemory.Mechanism.
   enum Type {
     // The SharedMemoryHandle is backed by a POSIX fd.
     POSIX,
     // The SharedMemoryHandle is backed by the Mach primitive "memory object".
     MACH,
   };
+  static const int TypeMax = 2;
 
   // The format that should be used to transmit |Type| over the wire.
   typedef int TypeWireFormat;