Cancel JavaScript dialogs when an interstitial appears.

BUG=295695
TEST=See bug for repro steps.

Review URL: https://chromiumcodereview.appspot.com/24360011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225026 0039d316-1c4b-4281-b951-d872f2087c98
WebPreferences WebContentsImpl::GetWebkitPrefs() {
  GURL url = controller_.GetActiveEntry()
      ? controller_.GetActiveEntry()->GetURL() : GURL::EmptyGURL();
  return GetWebkitPrefs(GetRenderViewHost(), url);
}

WebPreferences WebContentsImpl::GetWebkitPrefs() {
  GURL url = controller_.GetActiveEntry()
      ? controller_.GetActiveEntry()->GetURL() : GURL::EmptyGURL();
  return GetWebkitPrefs(GetRenderViewHost(), url);
}
