trace: use standard library context

Fixes golang/go#20056

Change-Id: I139d531ae87273fdc2409d29d63bb9d67680f30a
Reviewed-on: https://go-review.googlesource.com/41290
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

trace/trace.go

@@ -64,6 +64,7 @@ package trace // import "golang.org/x/net/trace"
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"html/template"
 	"io"
@@ -77,7 +78,6 @@ import (
 	"sync/atomic"
 	"time"
 
-	"golang.org/x/net/context"
 	"golang.org/x/net/internal/timeseries"
 )