[ContextEGL] Synchronize state only when first making current ContextEGL::onMakeCurrent() can be called when this context is already current, which introduces complexities for the case where this context is wrapping an external context. This CL ensures that in this case we save state from the native context only when first transitioning this context to be current. To test: Exercise the repro steps in crbug.com/1434657: No assert should go off. Bug: chromium:1434657 Change-Id: I9690a76917fcb6d0fa31361bfbd3766bb6074d41 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4483695 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org> Auto-Submit: Colin Blundell <blundell@chromium.org>