Remove Internals::olderShadowRoot

We don't need Internals::olderShadowRoot anymore.

Although Internals::olderShadowRoot doesn't expose user-agent shadow roots, 
existing tests which use Internals::olderShadowRoot don't depend on this behavior.

ShadowRoot.olderShadowRoot works fine for all existing tests.

BUG=None

Review URL: https://codereview.chromium.org/142443008

git-svn-id: svn://svn.chromium.org/blink/trunk@166584 bbb929c8-8fbe-4397-9dbb-9b2b20218538