Enable creation of offscreen contexts which own their backing surface.

ui/gl/init/gl_factory.h

Index: ui/gl/init/gl_factory.h
diff --git a/ui/gl/init/gl_factory.h b/ui/gl/init/gl_factory.h
index be8b2441c46e65604ece9b0d7cd7affff69b62d8..029359e8c74014bbbb22504ab6948ca8334a48c0 100644
--- a/ui/gl/init/gl_factory.h
+++ b/ui/gl/init/gl_factory.h
@@ -70,6 +70,10 @@ GL_INIT_EXPORT scoped_refptr<GLSurface> CreateSurfacelessViewGLSurface(
 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