logdog: Add frame read/write library.

common/logdog/frame/doc.go

+// Copyright 2015 The Chromium 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 frame implements a basic data frame protocol, as well as a reader
+// and writer for the protocol.
+//
+// LogDog uses frames to contain chunks of data (usually serialized protobufs).
+// The frame protocol consists of a series of back-to-back frames containing
+// opaque data types.
+//
+// Each frame begins with a Uvarint (
+// http://golang.org/pkg/encoding/binary/#Uvarint) containing the size of the
+// frame, followed by that many bytes of frame data.
+//
+// The frame protocol does not handle data integrity; that is left to the
+// outer protocol or medium which uses the frame.
+package frame