Clean up WebCustomElement and other FIXMEs post Chromium r218769

BUG=273126
TEST=fast/dom/custom, browser_tests --gtest_filter=WebViewTest.*

Review URL: https://chromiumcodereview.appspot.com/23129029

git-svn-id: svn://svn.chromium.org/blink/trunk@156734 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add a webexposed test for Custom Elements.

The expected result in virtual/stable/webexposed ensures that Custom
Elements don't affect the processing of elements by default.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/23417002

git-svn-id: svn://svn.chromium.org/blink/trunk@156735 bbb929c8-8fbe-4397-9dbb-9b2b20218538