If an image was used by a different context previously, it's stored serial
is not relevent.  We always know that any work involving the swap chain image
has been completed by the time we aquire a new one.

BUG=angleproject:2464

Change-Id: I0b12b9ff4b9a68593d661344d2d1e2c9d94e4755
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1592043Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>