Make OffscreenCanvas Webgl Context work with preserveDrawingBuffer flag

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

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1637 matching lines @@
                            HTMLCanvasElement*,
                            GLuint,
                            GLenum,
                            GLenum,
                            GLint,
                            GLint,
                            GLint,
                            const IntRect& sourceSubRectangle);
   void texImageBitmapByGPU(ImageBitmap*, GLuint, GLenum, GLenum, GLint, bool);
 
+  sk_sp<SkImage> makeImageSnapshot(SkImageInfo&);
   const unsigned m_version;
 
   bool isPaintable() const final { return drawingBuffer(); }
 };
 
 DEFINE_TYPE_CASTS(WebGLRenderingContextBase,
                   CanvasRenderingContext,
                   context,
                   context->is3d(),
                   context.is3d());
 
 }  // namespace blink
 
 WTF_ALLOW_MOVE_INIT_AND_COMPARE_WITH_MEM_FUNCTIONS(
     blink::WebGLRenderingContextBase::TextureUnitState);
 
 #endif  // WebGLRenderingContextBase_h