Move getBufferSubDataAsync to an extension

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

Index: third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.h
diff --git a/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.h b/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.h
index 88e7a1c35e98c9261c63136c8d5c054e29d2e61c..b2d07a276c2a4c45cc753b57b66564f8cab6d023 100644
--- a/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.h
+++ b/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.h
@@ -5,7 +5,6 @@
 #ifndef WebGL2RenderingContextBase_h
 #define WebGL2RenderingContextBase_h
 
-#include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/TraceWrapperMember.h"
 #include "modules/webgl/WebGLExtension.h"
 #include "modules/webgl/WebGLRenderingContextBase.h"
@@ -17,6 +16,7 @@ class WebGLTexture;
 
 class WebGLActiveInfo;
 class WebGLBuffer;
+class WebGLGetBufferSubDataAsync;
 class WebGLGetBufferSubDataAsyncCallback;
 class WebGLProgram;
 class WebGLQuery;
@@ -46,12 +46,6 @@ class WebGL2RenderingContextBase : public WebGLRenderingContextBase {
 
   void copyBufferSubData(GLenum, GLenum, long long, long long, long long);
   void getBufferSubData(GLenum, long long, DOMArrayBufferView*, GLuint, GLuint);
-  ScriptPromise getBufferSubDataAsync(ScriptState*,
-                                      GLenum target,
-                                      GLintptr srcByteOffset,
-                                      DOMArrayBufferView*,
-                                      GLuint dstOffset,
-                                      GLuint length);
 
   void registerGetBufferSubDataAsyncCallback(
       WebGLGetBufferSubDataAsyncCallback*);
@@ -747,6 +741,7 @@ class WebGL2RenderingContextBase : public WebGLRenderingContextBase {
 
  protected:
   friend class V8WebGL2RenderingContext;
+  friend class WebGLGetBufferSubDataAsync;
 
   WebGL2RenderingContextBase(
       HTMLCanvasElement*,