<webview>: Do not show blocked_by_client error page for guest.

<webview>: Do not show blocked_by_client error page for guest.

When a request to load a top level frame inside webview is cancelled by
using <webview>'s WebRequest API, by default we load
"This webpage is blocked by an extension" error page.
This CL avoids loading error page's html in guest render view.
We already fire loadabort event in this case.

BUG=284741
Test=With webrequest API, a webview containing:
<a href="http://foo">Foo</a>

When clicked on the link, if embedder wants to prevent navigating the <webview>
to foo, does so using webRequest API to cancel:

<webview>.request.onBeforeRequest.addListener(function(details) {
  if (details.type == 'main_frame' && details.url == 'http://foo' {
    window.open('...');
    return {cancel: true};
  }
  return {cancel: 'false'};
},
{urls: ['<all_urls>']},
['blocking']);

The "return {cancel:true}" won't load the mentioned error page anymore in
the webview.

Review URL: https://chromiumcodereview.appspot.com/23856006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@223769 0039d316-1c4b-4281-b951-d872f2087c98