runtime: add Frames example

Based on sample code from iant.

Fixes #18788.

Change-Id: I6bb33ed05af2538fbde42ddcac629280ef7c00a6
Reviewed-on: https://go-review.googlesource.com/36892
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/example_test.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime_test
+
+import (
+	"fmt"
+	"runtime"
+)
+
+func ExampleFrames() {
+	c := func() {
+		pc := make([]uintptr, 5)
+		n := runtime.Callers(0, pc)
+		if n == 0 {
+			return
+		}
+
+		frames := runtime.CallersFrames(pc[:n])
+		var frame runtime.Frame
+		more := true
+		for more {
+			frame, more = frames.Next()
+			fmt.Printf("- more:%v | %s\n", more, frame.Function)
+		}
+	}
+
+	b := func() { c() }
+	a := func() { b() }
+
+	a()
+	// Output:
+	// - more:true | runtime.Callers
+	// - more:true | runtime_test.ExampleFrames.func1
+	// - more:true | runtime_test.ExampleFrames.func2
+	// - more:true | runtime_test.ExampleFrames.func3
+	// - more:false | runtime_test.ExampleFrames
+}