ui: Enable virtual contexts for linux

ui/gl/gl_surface_x11.cc

Index: ui/gl/gl_surface_x11.cc
diff --git a/ui/gl/gl_surface_x11.cc b/ui/gl/gl_surface_x11.cc
index a384147cb2feb188cd3b3acde518f308e70b8511..f7d6a41f3b637109c2dc16df3cbafebf9685be50 100644
--- a/ui/gl/gl_surface_x11.cc
+++ b/ui/gl/gl_surface_x11.cc
@@ -319,7 +319,8 @@ scoped_refptr<GLSurface> GLSurface::CreateOffscreenGLSurface(
       return surface;
     }
     case kGLImplementationDesktopGL: {
-      scoped_refptr<GLSurface> surface(new PbufferGLSurfaceGLX(size));
+      scoped_refptr<GLSurface> surface(
+          new UnmappedNativeViewGLSurfaceGLX(size));
       if (!surface->Initialize())
         return NULL;