Speculative fix for crashes during flush on Mac. Mirrors the
flush_on_framebuffer_change workaround in Chrome from
http://crbug.com/783979

Bug: chromium:1181068
Change-Id: Ic84b1bfc44babf27016b306393da308d504d7fd9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2859984Reviewed-by: Peng Huang <penghuang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>