Mojo: Start writing some unit tests for mojo::system::Channel.

mojo/system/raw_channel.h

Index: mojo/system/raw_channel.h
diff --git a/mojo/system/raw_channel.h b/mojo/system/raw_channel.h
index b4571f8428a6f2e15018f91fc3e92c4fab67fe07..607a7761f7696f859ee74258e89778ee41f97b86 100644
--- a/mojo/system/raw_channel.h
+++ b/mojo/system/raw_channel.h
@@ -104,7 +104,7 @@ class MOJO_SYSTEM_IMPL_EXPORT RawChannel {
     IO_PENDING
   };
 
-  class ReadBuffer {
+  class MOJO_SYSTEM_IMPL_EXPORT ReadBuffer {
    public:
     ReadBuffer();
     ~ReadBuffer();
@@ -124,7 +124,7 @@ class MOJO_SYSTEM_IMPL_EXPORT RawChannel {
     DISALLOW_COPY_AND_ASSIGN(ReadBuffer);
   };
 
-  class WriteBuffer {
+  class MOJO_SYSTEM_IMPL_EXPORT WriteBuffer {
    public:
     struct Buffer {
       const char* addr;