http: fix build on Go 1.6 and below

https://golang.org/cl/29965 adding ClientConn.Ping support introduced
a build failure on Go versions before 1.7.

Fixes golang/go#17286

Change-Id: Ibfb565e7d823a436e58dc833973d7bdb41b7de5a
Reviewed-on: https://go-review.googlesource.com/30071
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

http2/not_go17.go

@@ -12,7 +12,10 @@ import (
 	"net/http"
 )
 
-type contextContext interface{}
+type contextContext interface {
+	Done() <-chan struct{}
+	Err() error
+}
 
 type fakeContext struct{}