<webview>: Report a better message if plugin not loaded.

Currently, if <webview> is navigated to a page with an unsupported plugin,
it'll show a "Looking for plugin..." placeholder indefinitely. With this CL,
if there is an unsupport plugin then the message: "No plugin available to display
this content." will be shown instead.

BUG=345348

Review URL: https://codereview.chromium.org/175053004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252828 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252828