Context has the invariant that if it is not current anywhere, mCurrentSuface is
null.  If makeCurrent fails in ContextImpl::onMakeCurrent, the surface is not
reset, breaking this invariant.

BUG=angleproject:3414

Change-Id: Icb13c8e2d1edd98d87a8af091a3fbacebadb395c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1592064Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>