We were calling context->releaseSurface on a context that was no longer
current, this somtimes caused incorrect framebuffers to be deleted.

BUG=angleproject:2464

Change-Id: I6859b5739a509c1343901f8c0eef1fdd1598cea1
Reviewed-on: https://chromium-review.googlesource.com/1108087Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>