Add dictionary for OfflineAudioContext constructor

third_party/WebKit/Source/modules/webaudio/OfflineAudioContextOptions.idl

Index: third_party/WebKit/Source/modules/webaudio/OfflineAudioContextOptions.idl
diff --git a/third_party/WebKit/Source/modules/webaudio/OfflineAudioContextOptions.idl b/third_party/WebKit/Source/modules/webaudio/OfflineAudioContextOptions.idl
new file mode 100644
index 0000000000000000000000000000000000000000..ecae9b54077ae1fafc74173f7c31a89d0023114f
--- /dev/null
+++ b/third_party/WebKit/Source/modules/webaudio/OfflineAudioContextOptions.idl
@@ -0,0 +1,9 @@
+// Copyright 2017 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.
+
+dictionary OfflineAudioContextOptions {
+  unsigned long numberOfChannels = 1;
+  required unsigned long length;
+  required float sampleRate;
+};