Remove Element::uaShadowElementById.

The function has no benefit. userAgentShadowRoot is always non-null
whenever this function is used.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23162003

git-svn-id: svn://svn.chromium.org/blink/trunk@156079 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove support for [IsIndex] IDL extended attribute

Remove support for [IsIndex] IDL extended attribute as it is no longer
needed and it increases the complexity of the bindings generator.

BUG=237739

Review URL: https://chromiumcodereview.appspot.com/22962003

git-svn-id: svn://svn.chromium.org/blink/trunk@156080 bbb929c8-8fbe-4397-9dbb-9b2b20218538