It was possible for ANGLE to call eglMakeCurrent with EGL_NO_SURFACE and
a valid context when the client called eglMakeCurrent with
EGL_NO_SURFACE and EGL_NO_CONTEXT.  Fix this by always binding a surface
when the driver has no native surfaceless support.

Don't expose the surfaceless extension when it's not possible to support
it (unvirtualized contexts and no native surfaceless support).

BUG=860800
BUG=angleproject:2464

Change-Id: Id8af9638d4356dbd710c453c9f196b9f25a2bbf9
Reviewed-on: https://chromium-review.googlesource.com/1131555Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>