net/http/pprof: run GC for /debug/pprof/heap?gc=1

We force runtime.GC before WriteHeapProfile with -test.heapprofile.
Make it possible to do the same with the HTTP interface.

Some servers only run a GC every few minutes.
On such servers, the heap profile will be a few minutes stale,
which may be too old to be useful.

Requested by private mail.

LGTM=dvyukov
R=dvyukov
CC=golang-codereviews
https://golang.org/cl/161990043

src/net/http/pprof/pprof.go

@@ -162,6 +162,10 @@ func (name handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		fmt.Fprintf(w, "Unknown profile: %s\n", name)
 		return
 	}
+	gc, _ := strconv.Atoi(r.FormValue("gc"))
+	if name == "heap" && gc > 0 {
+		runtime.GC()
+	}
 	p.WriteTo(w, debug)
 	return
 }