This test will ensure the backbuffer is also properly resized when
the window is resized.

Bug: angleproject:4167
Change-Id: I1d73f07379b4f05e453520e22ef1369f51182197
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1934311
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>