Try to re-enable webgl reftests

I can't reproduce locally the failure that produced them, and since the
builds that triggered them are deleted for some reason (like
http://build.servo.org/builders/linux-dev/builds/419), it could be a
good moment to troubleshoot it.

If merged closes #7931

tests/ref/basic.list

@@ -107,13 +107,12 @@ resolution=800x600 == viewport_percentage_vw_vh.html viewport_percentage_vw_vh_a
 # resolution=600x800 == viewport_percentage_vw_vh.html viewport_percentage_vw_vh_b.html
 prefs:"layout.viewport.enabled" == viewport_rule.html viewport_rule_ref.html
 
-# https://github.com/servo/servo/issues/7931
-# == webgl-context/clearcolor.html webgl-context/clearcolor_ref.html
-# == webgl-context/draw_arrays_simple.html webgl-context/draw_arrays_simple_ref.html
-# == webgl-context/tex_image_2d_canvas.html webgl-context/tex_image_2d_canvas_ref.html
-# == webgl-context/tex_image_2d_canvas2d.html webgl-context/tex_image_2d_canvas_ref.html
-# == webgl-context/tex_image_2d_canvas_no_context.html webgl-context/tex_image_2d_canvas_no_context_ref.html
-# == webgl-context/tex_image_2d_simple.html webgl-context/tex_image_2d_simple_ref.html
+== webgl-context/clearcolor.html webgl-context/clearcolor_ref.html
+== webgl-context/draw_arrays_simple.html webgl-context/draw_arrays_simple_ref.html
+== webgl-context/tex_image_2d_canvas.html webgl-context/tex_image_2d_canvas_ref.html
+== webgl-context/tex_image_2d_canvas2d.html webgl-context/tex_image_2d_canvas_ref.html
+== webgl-context/tex_image_2d_canvas_no_context.html webgl-context/tex_image_2d_canvas_no_context_ref.html
+== webgl-context/tex_image_2d_simple.html webgl-context/tex_image_2d_simple_ref.html
 
 flaky_macos == white_space_intrinsic_sizes_a.html white_space_intrinsic_sizes_ref.html