implement getBufferSubDataAsync prototype

third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.idl

 // 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.
 
 typedef long long      GLint64;
 
 [
     RuntimeEnabled=UnsafeES3APIs,
     NoInterfaceObject,
 ] interface WebGL2RenderingContextBase {
@@ skipping 264 matching lines @@
     const GLint TIMEOUT_IGNORED                                = -1;
 
     /* WebGL-specific enums */
     const GLenum MAX_CLIENT_WAIT_TIMEOUT_WEBGL                 = 0x9247;
 
     /* Buffer objects */
     void bufferData(GLenum target, ArrayBufferView srcData, GLenum usage, GLuint srcOffset, optional GLuint length = 0);
     void bufferSubData(GLenum target, GLintptr dstByteOffset, ArrayBufferView srcData, GLuint srcOffset, optional GLuint length = 0);
     void copyBufferSubData(GLenum readTarget, GLenum writeTarget, GLintptr readOffset, GLintptr writeOffset, GLsizeiptr size);
     void getBufferSubData(GLenum target, GLintptr srcByteOffset, ArrayBufferView dstData, optional GLuint dstOffset = 0, optional GLuint length = 0);
+    [CallWith=ScriptState, RuntimeEnabled=ExperimentalCanvasFeatures] Promise<ArrayBufferView> getBufferSubDataAsync(GLenum target, GLintptr srcByteOffset, ArrayBufferView dstData, optional GLuint dstOffset = 0, optional GLuint length = 0);
 
     /* Framebuffer objects */
     void blitFramebuffer(GLint srcX0, GLint srcY0, GLint srcX1, GLint srcY1, GLint dstX0, GLint dstY0, GLint dstX1, GLint dstY1, GLbitfield mask, GLenum filter);
     void framebufferTextureLayer(GLenum target, GLenum attachment, WebGLTexture? texture, GLint level, GLint layer);
     [CallWith=ScriptState] any getInternalformatParameter(GLenum target, GLenum internalformat, GLenum pname);
     void invalidateFramebuffer(GLenum target, sequence<GLenum> attachments);
     void invalidateSubFramebuffer (GLenum target, sequence<GLenum> attachments, GLint x, GLint y, GLsizei width, GLsizei height);
     void readBuffer(GLenum mode);
 
     /* Renderbuffer objects */
@@ skipping 149 matching lines @@
     /* Vertex Array Objects */
     WebGLVertexArrayObject createVertexArray();
     void deleteVertexArray(WebGLVertexArrayObject? vertexArray);
     GLboolean isVertexArray(WebGLVertexArrayObject? vertexArray);
     void bindVertexArray(WebGLVertexArrayObject? vertexArray);
 
     /* Reading */
     void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLintptr offset);
 };
 WebGL2RenderingContextBase implements WebGLRenderingContextBase;