[WebView] Add onPageCommitVisible callback.

This provides a convenience callback, onPageCommitVisible, that
makes use of the visual state listener API. It is called at the
earliest point at which the next draw will not render contents from
the previously loaded page.

BUG=457184, 458566

Review URL: https://codereview.chromium.org/984603002

Cr-Commit-Position: refs/heads/master@{#320361}
12 files changed