Refactor Nacl to remove assumptions about SharedMemoryHandle.

components/nacl/browser/nacl_process_host.cc

Index: components/nacl/browser/nacl_process_host.cc
diff --git a/components/nacl/browser/nacl_process_host.cc b/components/nacl/browser/nacl_process_host.cc
index 4c719fd5ded2ac1a181bbdfb66d82448b8852b53..742e773d7ac9275bca66d7aec626c41465256b0d 100644
--- a/components/nacl/browser/nacl_process_host.cc
+++ b/components/nacl/browser/nacl_process_host.cc
@@ -946,7 +946,10 @@ bool NaClProcessHost::StartNaClExecution() {
       DLOG(ERROR) << "Failed to allocate memory buffer";
       return false;
     }
-    base::ScopedFD memory_fd(dup(memory_buffer.handle().fd));
+    base::SharedMemoryHandle duped_handle =
+        base::SharedMemory::DuplicateHandle(memory_buffer.handle());
+    base::ScopedFD memory_fd(
+        base::SharedMemory::GetFdFromSharedMemoryHandle(duped_handle));

[Mark Seaborn, 2015/06/04 20:17:55]

How will this help you switch to having SharedMemoryHandle be a Mach port on Mac
OS X?  This code is still assuming that you can convert a SharedMemoryHandle to
an FD.  Will it be possible to convert a Mach port to an FD?

Have you got follow-on changes prepared which do that?

[erikchen, 2015/06/04 20:25:42]

I have a CL that changes SharedMemoryHandle to be a class on OSX, which is a
necessary first step to making SharedMemoryHandle backed by Mach primitives. 

https://codereview.chromium.org/1163943004/

Before I can make SharedMemoryHandle a class, I need to update all existing
references to SharedMemoryHandle that assume that SharedMemoryHandle is
typedefed to FileDescriptor. This is one of those locations.

If your curious, my next high level steps are:
1) Make SharedMemoryHandle a class that wraps POSIX fd.
2) Make SharedMemoryHandle support both POSIX fd and Mach primitives.
3) Migrate consumers of base::SharedMemory to use Mach primitives.

[Mark Seaborn, 2015/06/04 20:45:01]

Which cases will require you to support storing an FD in SharedMemoryHandle on
Mac?  Do you intend to support that indefinitely?  Is it only NaCl that requires
this, or are there other cases?

Presumably you would change base::SharedMemory::Create() so that the caller can
specify whether to create an FD or a Mach port?

     if (!memory_fd.is_valid()) {
       DLOG(ERROR) << "Failed to dup() a file descriptor";
       return false;