Enable creation of offscreen contexts which own their backing surface.

ui/gl/init/gl_factory.h

 // 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.
 
 #ifndef UI_GL_INIT_GL_FACTORY_H_
 #define UI_GL_INIT_GL_FACTORY_H_
 
 #include <vector>
 
 #include "base/memory/ref_counted.h"
@@ skipping 52 matching lines @@
 // be presented as an overlay. If surfaceless mode is not supported or
 // enabled it will return a null pointer.
 GL_INIT_EXPORT scoped_refptr<GLSurface> CreateSurfacelessViewGLSurface(
     gfx::AcceleratedWidget window);
 #endif  // defined(USE_OZONE)
 
 // Creates a GL surface used for offscreen rendering.
 GL_INIT_EXPORT scoped_refptr<GLSurface> CreateOffscreenGLSurface(
     const gfx::Size& size);
 
+// Creates a GL surface used for offscreen rendering without initializing it.
+GL_INIT_EXPORT scoped_refptr<GLSurface> CreateUninitializedOffscreenGLSurface(
+    const gfx::Size& size);

[piman, 2016/11/16 00:44:02]

So, it sounds like the only reason you want to not initialize is so that you can
specify the format. Maybe a better API would be
CreateOffscreenGLSurfaceWithFormat(const gfx::Size& size, GLSurface::Format
format).

Separately, it would be nice to refactor the logic a bit so that
CreateOffscreenGLSurface(size) is equivalent to
CreateOffscreenGLSurfaceWithFormat(size, SURFACE_DEFAULT). This could be a
separate change:
- make SURFACE_DEFAULT its own value (as opposed to SURFACE_ARGB8888)
- make GLSurface::Initialize() (the 0 parameter version) non-virtual and remove
overloads
- in each of the GLSurface::Initialize(GLSurface::Format), explicitly handle
SURFACE_DEFAULT and convert to the right format (i.e. SURFACE_ARGB8888 almost
everywhere, except PbufferGLSurfaceEGL and SurfacelessEGL which have a special
Initialize() overload - that logic would move to Initialize(GLSurface::Format))

[klausw, 2017/01/05 02:17:43]

I briefly tried this but it caused a bunch of complications,
it would need careful conversion in GetFormat and other places
to ensure that comparisons would work as expected. The new
approach uses a GLSurfaceFormat class and moves non-default
properties to appropriate subtype initialization.
Done.
Handled a bit differently, using merging of properties on the
GLSurfaceFormat class instead of directly overwriting.

+
 }  // namespace init
 }  // namespace gl
 
 #endif  // UI_GL_INIT_GL_FACTORY_H_