GitLab

Currently, when a poster image is specified that's 404 Not Found, or
too small, the placeholder can become too small and unclickable.

This change forces the placeholder HTML to expand to the size of the
WebViewPlugin.

BUG=475653

Review URL: https://codereview.chromium.org/1085993003

Cr-Commit-Position: refs/heads/master@{#326897}