media-sharing: Remove variable name conflicts from VDL generated Go code

The previous VDL generated Go code contained common variable
names, such as ctx, opts and err.  This meant that users couldn't
pick these names for their variables.  This CL fixes those cases.

Note that we already generate different code for exported
interfaces vs. unexported implementations.  The interfaces use
user-provided variable names, which is better for documentation,
while the implementations already avoid conflicts by using
generated variables names i0, i1, ... for input args, and
similarly o0, o1, ... for output args.

This CL ensures the generated interfaces use _ for non-user
provided variables, to avoid any possible naming conflict.

Fixes v.io/i/912

MultiPart: 6/7

Change-Id: I835bf9069c3076540c1d67c50088c6833ba33f76
diff --git a/go/src/v.io/x/media_sharing/media.vdl.go b/go/src/v.io/x/media_sharing/media.vdl.go
index 0bdd263..4c5c233 100644
--- a/go/src/v.io/x/media_sharing/media.vdl.go
+++ b/go/src/v.io/x/media_sharing/media.vdl.go
@@ -22,12 +22,12 @@
 	// the given URL.  The server will rely on the ContentType response
 	// header it gets when fetching the url to decide how to display
 	// the media.
-	DisplayUrl(ctx *context.T, url string, opts ...rpc.CallOpt) error
+	DisplayUrl(_ *context.T, url string, _ ...rpc.CallOpt) error
 	// DisplayBytes will cause the server to display whatever media is
 	// sent in the stream.  In the case of audio or movie media, the
 	// media should be played while the data is streaming.  The mediaType
 	// can be used by the server to decide how to display the media.
-	DisplayBytes(ctx *context.T, mediaType string, opts ...rpc.CallOpt) (MediaSharingDisplayBytesClientCall, error)
+	DisplayBytes(_ *context.T, mediaType string, _ ...rpc.CallOpt) (MediaSharingDisplayBytesClientCall, error)
 }
 
 // MediaSharingClientStub adds universal methods to MediaSharingClientMethods.
@@ -129,12 +129,12 @@
 	// the given URL.  The server will rely on the ContentType response
 	// header it gets when fetching the url to decide how to display
 	// the media.
-	DisplayUrl(ctx *context.T, call rpc.ServerCall, url string) error
+	DisplayUrl(_ *context.T, _ rpc.ServerCall, url string) error
 	// DisplayBytes will cause the server to display whatever media is
 	// sent in the stream.  In the case of audio or movie media, the
 	// media should be played while the data is streaming.  The mediaType
 	// can be used by the server to decide how to display the media.
-	DisplayBytes(ctx *context.T, call MediaSharingDisplayBytesServerCall, mediaType string) error
+	DisplayBytes(_ *context.T, _ MediaSharingDisplayBytesServerCall, mediaType string) error
 }
 
 // MediaSharingServerStubMethods is the server interface containing
@@ -146,12 +146,12 @@
 	// the given URL.  The server will rely on the ContentType response
 	// header it gets when fetching the url to decide how to display
 	// the media.
-	DisplayUrl(ctx *context.T, call rpc.ServerCall, url string) error
+	DisplayUrl(_ *context.T, _ rpc.ServerCall, url string) error
 	// DisplayBytes will cause the server to display whatever media is
 	// sent in the stream.  In the case of audio or movie media, the
 	// media should be played while the data is streaming.  The mediaType
 	// can be used by the server to decide how to display the media.
-	DisplayBytes(ctx *context.T, call *MediaSharingDisplayBytesServerCallStub, mediaType string) error
+	DisplayBytes(_ *context.T, _ *MediaSharingDisplayBytesServerCallStub, mediaType string) error
 }
 
 // MediaSharingServerStub adds universal methods to MediaSharingServerStubMethods.