Fixed webview_plugin placeholders not rendering.
The plugin placeholder was loading into a WebView, which had viewport
enabled so the placeholder was loading the page at the 980px layout width.
For some reason this caused it not to render at all. Setting the plugin WebView
to use the FixedLayoutSize pegged to the WebView size fixes the problem.
BUG=320621
Review URL: https://codereview.chromium.org/
81373002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236645 0039d316-1c4b-4281-b951-d872f2087c98