Rename ClosedShadowRoot to UserAgentShadowRoot

      This is a partial revert of batch renaming in https://codereview.chromium.org/935283002
because the name of ClosedShadowRoot caused the confusion.

Instead of batch renaming, I'll introduce the name of &quotclosed&quot in a more incremental way so that
&quotClosedShadowRoot&quot and &quotUserAgentShadowRoot&quot can co-exist, with the different meanings, which doesn't upset the existing code.

BUG=477203

Review URL: https://codereview.chromium.org/1091473002

git-svn-id: svn://svn.chromium.org/blink/trunk@193769 bbb929c8-8fbe-4397-9dbb-9b2b20218538