[chrome.displaySource] Add a base class for audio and video encoders.

extensions/renderer/api/display_source/wifi_display/wifi_display_media_encoder.cc

+// Copyright 2016 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.
+
+#include "extensions/renderer/api/display_source/wifi_display/wifi_display_media_encoder.h"
+
+#include "base/logging.h"
+#include "media/base/bind_to_current_loop.h"
+
+namespace extensions {
+
+WiFiDisplayEncodedUnit::WiFiDisplayEncodedUnit(
+    std::string data,
+    base::TimeTicks reference_timestamp,
+    bool key_frame)
+    : data(std::move(data)), pts(reference_timestamp), key_frame(key_frame) {}
+
+WiFiDisplayEncodedUnit::WiFiDisplayEncodedUnit(
+    std::string data,
+    base::TimeTicks reference_timestamp,
+    base::TimeTicks encode_timestamp,
+    bool key_frame)
+    : WiFiDisplayEncodedUnit(std::move(data), reference_timestamp, key_frame) {
+  if (encode_timestamp >= reference_timestamp) {
+    dts = reference_timestamp - (encode_timestamp - reference_timestamp);
+    DCHECK_LE(dts, pts);
+  }
+}
+
+WiFiDisplayMediaEncoder::WiFiDisplayMediaEncoder() = default;
+WiFiDisplayMediaEncoder::~WiFiDisplayMediaEncoder() = default;
+
+void WiFiDisplayMediaEncoder::SetCallbacks(
+    const EncodedUnitCallback& encoded_callback,
+    const base::Closure& error_callback) {
+  // This is not thread-safe if encoding has been started thus allow
+  // this to be called only once.
+  DCHECK(encoded_callback_.is_null() && error_callback_.is_null());
+  encoded_callback_ = media::BindToCurrentLoop(encoded_callback);
+  error_callback_ = media::BindToCurrentLoop(error_callback);
+}
+
+}  // namespace extensions