pipe2browser: Move context.T out of rpc.ServerCall.

The purpose of this change is to make our usage of context.T more
consistent; it was a bit lame that we gave guidance to never wrap
context.T in another type, but were doing it ourselves.  The JS
code is also being changed to follow this convention (in separate
CLs), so we'll be consistent between Go and JS as well.

The server implementation used to look like this:
func (*impl) Foo(call rpc.ServerCall, ...)

Now it looks like this:
func (*impl) Foo(ctx *context.T, call rpc.ServerCall, ...)

Also added a ServerCall.Security() function, which returns the
security.Call.  The security.Call is still embedded inside
context.T for now; a subsequent change will remove it from
context.T and add an explicit security.Call argument where
necessary.  That's a separate CL since some of the choices may be
more controversial, and it's a smaller set of changes.

Change-Id: I7040035486f69cff3aaf215e3a9d8e797b79c6cf
MultiPart: 6/8
diff --git a/go/src/v.io/x/p2b/vdl/p2b.vdl.go b/go/src/v.io/x/p2b/vdl/p2b.vdl.go
index f5859da..54894b3 100644
--- a/go/src/v.io/x/p2b/vdl/p2b.vdl.go
+++ b/go/src/v.io/x/p2b/vdl/p2b.vdl.go
@@ -127,7 +127,7 @@
 	// Pipe creates a bidirectional pipe between client and viewer
 	// service, returns total number of bytes received by the service
 	// after streaming ends
-	Pipe(ViewerPipeServerCall) (*vdl.Value, error)
+	Pipe(*context.T, ViewerPipeServerCall) (*vdl.Value, error)
 }
 
 // ViewerServerStubMethods is the server interface containing
@@ -138,7 +138,7 @@
 	// Pipe creates a bidirectional pipe between client and viewer
 	// service, returns total number of bytes received by the service
 	// after streaming ends
-	Pipe(*ViewerPipeServerCallStub) (*vdl.Value, error)
+	Pipe(*context.T, *ViewerPipeServerCallStub) (*vdl.Value, error)
 }
 
 // ViewerServerStub adds universal methods to ViewerServerStubMethods.
@@ -170,8 +170,8 @@
 	gs   *rpc.GlobState
 }
 
-func (s implViewerServerStub) Pipe(call *ViewerPipeServerCallStub) (*vdl.Value, error) {
-	return s.impl.Pipe(call)
+func (s implViewerServerStub) Pipe(ctx *context.T, call *ViewerPipeServerCallStub) (*vdl.Value, error) {
+	return s.impl.Pipe(ctx, call)
 }
 
 func (s implViewerServerStub) Globber() *rpc.GlobState {