Add support for MojoChannel field trial

ipc/mojo/ipc_channel_mojo.cc

Index: ipc/mojo/ipc_channel_mojo.cc
diff --git a/ipc/mojo/ipc_channel_mojo.cc b/ipc/mojo/ipc_channel_mojo.cc
index 8832d8d288c39a81ce670b4b5e385455c614840a..a0dabadcdda218ee977985694b97451d7d1a4491 100644
--- a/ipc/mojo/ipc_channel_mojo.cc
+++ b/ipc/mojo/ipc_channel_mojo.cc
@@ -199,13 +199,6 @@ MojoResult UnwrapAttachment(mojom::SerializedHandlePtr handle,
 //------------------------------------------------------------------------------
 
 // static
-bool ChannelMojo::ShouldBeUsed() {
-  // TODO(rockot): Investigate performance bottlenecks and hopefully reenable
-  // this at some point. http://crbug.com/500019
-  return false;
-}
-
-// static
 scoped_ptr<ChannelMojo> ChannelMojo::Create(
     mojo::ScopedMessagePipeHandle handle,
     Mode mode,