Rename CustomElementUpgradeCandidateMap::DestructionObserverMap.

While they observe destruction, they more specifically observe
*element* destruction, so rename this ElementObserverMap. This is both
more specific and more flexible if this needs to observe other parts
of the element lifecycle in future.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/22847004

git-svn-id: svn://svn.chromium.org/blink/trunk@156063 bbb929c8-8fbe-4397-9dbb-9b2b20218538