Wrap Custom Elements in isolated worlds at the undifferentiated prototype.

How Custom Elements should interact with isolated worlds hasn't been
thought out yet. For now, we follow the Prime Directive and don't
interfere with isolated worlds: Custom Elements accessed from isolated
worlds have ordinary prototypes. Previously accessing a custom element
upgraded in the main world from an isolated world caused the whole
enterprise to crash.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/20897002

git-svn-id: svn://svn.chromium.org/blink/trunk@155059 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Unskip 'security/block-test.html'.

The test is skipped because of behavioral differences between WebKit and
Chromium's network stack. Because we never hit the network for invalid
port numbers, we don't generate a resource load callback. We do, however
now properly dispatch 'error' events for the failed loads.

This patch mildly rewrites the test to dump console messages for each
failed load so that each item is visibly listed in the output. We should
be able to drop the platform-specific results as well. Yay!

BUG=39423

Review URL: https://chromiumcodereview.appspot.com/20919002

git-svn-id: svn://svn.chromium.org/blink/trunk@155060 bbb929c8-8fbe-4397-9dbb-9b2b20218538