appcache ntp: Fallback to local, don't prerender.

GoogleBaseURL starts out as http and is only upgraded to https after
search domain check, so we'll try to load http://<newtabpage> and fail.
The failed load either 404s or shows the local new tab page, depending
on when the search domain check finishes. Instead, if the new tab url
isn't secure, prefer the the baked-in new tab page.

Also fall back to the local ntp if the user has no search
provider, if the user's search provider has no custom ntp,
or if the user disables search suggestions.

Also, don't prerender the old instant ntp anymore.

TEST=manual,unit
BUG=304184,304901,304397

Review URL: https://codereview.chromium.org/26024002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227460 0039d316-1c4b-4281-b951-d872f2087c98
Delete AppShimHostManager on UI thread. (Mac)

This has been crashing on shutdown due to a CHECK in
NotificationServiceImpl::RemoveObserver from:
- content::NotificationRegistrar::~NotificationRegistrar()
- apps::ExtensionAppShimHandler::~ExtensionAppShimHandler()
- AppShimHostManager::~AppShimHostManager()
As per the comments in RemoveObserver, the fix is to delete the
NotificationRegistrar on the same thread that created it.

BUG=304197

Review URL: https://codereview.chromium.org/26398003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227461 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227461