runtime/pprof: actually use tag parameter

It's only ever called with the value it was using, but the code was
counterintuitive. Use the parameter instead, like the other funcs near
it.

Found by github.com/mvdan/unparam.

Change-Id: I45855e11d749380b9b2a28e6dd1d5dedf119a19b
Reviewed-on: https://go-review.googlesource.com/37893Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/pprof/proto.go

@@ -178,7 +178,7 @@ func (b *profileBuilder) pbMapping(tag int, id, base, limit, offset uint64, file
 	b.pb.int64Opt(tagMapping_Filename, b.stringIndex(file))
 	// TODO: Set any of HasInlineFrames, HasFunctions, HasFilenames, HasLineNumbers?
 	// It seems like they should all be true, but they've never been set.
-	b.pb.endMessage(tagProfile_Mapping, start)
+	b.pb.endMessage(tag, start)
 }
 
 // locForPC returns the location ID for addr.