ozone/platform/wayland: Drop WaylandWindow::GetWidget()

It was only used in tests and WaylandDisplay::{Add,Remove}Window, and
WaylandWindow can just provide the widget as an argument.

This way we can drop WaylandWindowInitializeTest, and just use the widget from
OnAcceleratedWidgetAvailable in all tests. This more closely matches reality.

Also, inline and rename GetSurface() -> surface() because it is a simple
accessor.

BUG=578890

Review URL: https://codereview.chromium.org/1742883002

Cr-Commit-Position: refs/heads/master@{#378733}
8 files changed