Move getBufferSubDataAsync to an extension

third_party/WebKit/Source/modules/webgl/WebGLGetBufferSubDataAsync.idl

Index: third_party/WebKit/Source/modules/webgl/WebGLGetBufferSubDataAsync.idl
diff --git a/third_party/WebKit/Source/modules/webgl/WebGLGetBufferSubDataAsync.idl b/third_party/WebKit/Source/modules/webgl/WebGLGetBufferSubDataAsync.idl
new file mode 100644
index 0000000000000000000000000000000000000000..f4cf48e7415d824ad2dd2eb82f3dc3b76b9046b2
--- /dev/null
+++ b/third_party/WebKit/Source/modules/webgl/WebGLGetBufferSubDataAsync.idl
@@ -0,0 +1,11 @@
+// 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.
+
+[
+    NoInterfaceObject,
+    DoNotCheckConstants,
+    SetWrapperReferenceFrom=canvas,
+] interface WebGLGetBufferSubDataAsync {
+    [CallWith=ScriptState] Promise<ArrayBufferView> getBufferSubDataAsync(GLenum target, GLintptr srcByteOffset, ArrayBufferView dstData, optional GLuint dstOffset = 0, optional GLuint length = 0);
+};