<webview>: Fix plugins on M32 beta and stable

BUG=322725

Review URL: https://codereview.chromium.org/104683004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238720 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238720