Give RenderWidget's visible viewport a size when running LayoutTests.

Layout tests were previously running with visible_viewport_size uninitialized,
causing layout tests to fail when running with virtual-viewport mode enabled.

BUG=

Review URL: https://codereview.chromium.org/649673005

Cr-Commit-Position: refs/heads/master@{#302088}
4 files changed