Support EXT_blend_func_extended in the GLES2 context BUG=angleproject:1379 Change-Id: Ibda6e84c0a7f86e838247cd5c538ef956dd786fe Reviewed-on: https://chromium-review.googlesource.com/346410 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Sami Väisänen <svaisanen@nvidia.com>