Split the element observation part out of CustomElementUpgradeCandidateMap.

Refactoring. No behavior change.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/22969003

git-svn-id: svn://svn.chromium.org/blink/trunk@156072 bbb929c8-8fbe-4397-9dbb-9b2b20218538