Support read-only duplicates of Mojo shared buffers.

mojo/edk/system/broker_posix.cc

Index: mojo/edk/system/broker_posix.cc
diff --git a/mojo/edk/system/broker_posix.cc b/mojo/edk/system/broker_posix.cc
index 91b9cb5a998c2cf8961b608038b726a9e80863af..3cbb953f1cb1ae6f594a262c61e55d85c35215c1 100644
--- a/mojo/edk/system/broker_posix.cc
+++ b/mojo/edk/system/broker_posix.cc
@@ -112,7 +112,8 @@ scoped_refptr<PlatformSharedBuffer> Broker::GetSharedBuffer(size_t num_bytes) {
                            BrokerMessageType::BUFFER_RESPONSE, 1,
                            &incoming_platform_handles)) {
     return PlatformSharedBuffer::CreateFromPlatformHandle(
-        num_bytes, ScopedPlatformHandle(incoming_platform_handles.front()));
+        num_bytes, false /* read_only */,
+        ScopedPlatformHandle(incoming_platform_handles.front()));
   }
 
   return nullptr;