Mojo: Abstract out write/writev vs send/sendmsg (for Unix domain sockets).

mojo/embedder/platform_channel_pair.h

Index: mojo/embedder/platform_channel_pair.h
diff --git a/mojo/embedder/platform_channel_pair.h b/mojo/embedder/platform_channel_pair.h
index f0629d6917ce28b726f858c7a53944c6cff12d8c..126bd3d98e14e9ce6d618889ed61e2ab7410c100 100644
--- a/mojo/embedder/platform_channel_pair.h
+++ b/mojo/embedder/platform_channel_pair.h
@@ -45,6 +45,11 @@ typedef base::FileHandleMappingVector HandlePassingInformation;
 // Note: |PlatformChannelPair()|, |PassClientHandleFromParentProcess()| and
 // |PrepareToPassClientHandleToChildProcess()| have platform-specific
 // implementations.
+//
+// Note: On POSIX platforms, to write to the "pipe", use
+// |PlatformChannel{Write,Writev}()| (from platform_channel_utils_posix.h)
+// instead of |write()|, |writev()|, etc. Otherwise, you have to worry about
+// platform differences in suppressing |SIGPIPE|.
 class MOJO_SYSTEM_IMPL_EXPORT PlatformChannelPair {
  public:
   PlatformChannelPair();