Implement SharedMemory::NewAnonymousReadOnly(contents).

base/memory/shared_memory_android.cc

 // Copyright (c) 2011 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.
 
 #include "base/memory/shared_memory.h"
 
 #include <sys/mman.h>
 
 #include "base/logging.h"
 #include "third_party/ashmem/ashmem.h"
@@ skipping 19 matching lines @@
     DLOG(ERROR) << "Shared memory creation failed";
     return false;
   }
 
   int err = ashmem_set_prot_region(mapped_file_,
                                    PROT_READ | PROT_WRITE | PROT_EXEC);
   if (err < 0) {
     DLOG(ERROR) << "Error " << err << " when setting protection of ashmem";
     return false;
   }
+
+  // Android doesn't appear to have a way to drop write access on an ashmem
+  // segment for a single descriptor.  http://crbug.com/320865
+  readonly_mapped_file_ = dup(mapped_file_);
+  if (-1 == readonly_mapped_file_) {
+    DPLOG(ERROR) << "dup() failed";
+    return false;
+  }
+
   requested_size_ = options.size;
 
   return true;
 }
 
 bool SharedMemory::Delete(const std::string& name) {
   // Like on Windows, this is intentionally returning true as ashmem will
   // automatically releases the resource when all FDs on it are closed.
   return true;
 }
 
 bool SharedMemory::Open(const std::string& name, bool read_only) {
   // ashmem doesn't support name mapping
   NOTIMPLEMENTED();
   return false;
 }
 
 }  // namespace base