Enable creation of offscreen contexts which own their backing surface.

ui/gl/gl_surface.h

Index: ui/gl/gl_surface.h
diff --git a/ui/gl/gl_surface.h b/ui/gl/gl_surface.h
index c0743f7f48547da5ae2ccacdc2f1ec9c6cef5851..ecbc1cb8ed1a3fc0808d5ff56eed3cb50e9abbd9 100644
--- a/ui/gl/gl_surface.h
+++ b/ui/gl/gl_surface.h
@@ -42,7 +42,9 @@ class GL_EXPORT GLSurface : public base::RefCounted<GLSurface> {
   // Minimum bit depth of surface.
   enum Format {
     SURFACE_ARGB8888,
+    SURFACE_ARGB8888_DEPTH24,
     SURFACE_RGB565,
+    SURFACE_RGB565_DEPTH24,

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

It seems like this won't scale to more depth bits or more surface formats.
We could either:
- explicitly add depth bits to GLSurface::Initialize
- change GLSurface::Initialize to take a GLSurfaceAttribs struct instead of
GLSurface::Format that can specify the color format, but also the depth and
stencil bits, MSAA samples, etc. over time as we need them.

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

See https://codereview.chromium.org/2616723002 for a new approach,
now using a GLSurfaceFormat class which replaces the custom format
types.

     SURFACE_OSMESA_BGRA,
     SURFACE_OSMESA_RGBA,
     SURFACE_SURFACELESS,