reland 2: Switch back to field trial.

base/memory/shared_memory_handle.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_MEMORY_SHARED_MEMORY_HANDLE_H_
 #define BASE_MEMORY_SHARED_MEMORY_HANDLE_H_
 
 #include "build/build_config.h"
 
 #if defined(OS_WIN)
@@ skipping 57 matching lines @@
  private:
   HANDLE handle_;
 
   // The process in which |handle_| is valid and can be used. If |handle_| is
   // invalid, this will be kNullProcessId.
   base::ProcessId pid_;
 };
 #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;
 
   // The default constructor returns an invalid SharedMemoryHandle.
   SharedMemoryHandle();
 
   // Constructs a SharedMemoryHandle backed by the components of a
   // FileDescriptor. The newly created instance has the same ownership semantics
   // as base::FileDescriptor. This typically means that the SharedMemoryHandle
@@ skipping 93 matching lines @@
       // Defaults to |false|.
       bool ownership_passes_to_ipc_;
     };
   };
 };
 #endif
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_SHARED_MEMORY_HANDLE_H_