Make PlatformChannelPair "dumb".

Now it's just a helper that creates two "connected" PlatformHandles
(modelled after mojo::MessagePipe, mojo::DataPipe, etc.). Passing one
handle to the child is now done manually.

(I want to move it to //mojo/edk/platform and maybe rename it too, but
I'll do that separately.)

R=vardhan@google.com

Review URL: https://codereview.chromium.org/1651183003 .
diff --git a/mojo/edk/system/channel_test_base.cc b/mojo/edk/system/channel_test_base.cc
index cc35b63..67d0a56 100644
--- a/mojo/edk/system/channel_test_base.cc
+++ b/mojo/edk/system/channel_test_base.cc
@@ -64,8 +64,8 @@
   CHECK(io_thread()->IsCurrentAndRunning());
 
   embedder::PlatformChannelPair channel_pair;
-  raw_channels_[0] = RawChannel::Create(channel_pair.PassServerHandle());
-  raw_channels_[1] = RawChannel::Create(channel_pair.PassClientHandle());
+  raw_channels_[0] = RawChannel::Create(channel_pair.handle0.Pass());
+  raw_channels_[1] = RawChannel::Create(channel_pair.handle1.Pass());
 }
 
 }  // namespace test