Remove custom element upgrades.

Remove custom element upgrades.

This removes the upgrade candidate machinery so now if an
element is created before it's registered it'll never get
turned into the correct type. created() callbacks and the
correct wrappers being created can stil happen async though
because certain operations are not safe to run script inside
of (ex. the parser or editing).

R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/e8fec7dafc9a5226234940697e6ced5bebeaf35d

[abarth-chromium, 2015-01-14 00:00:59]

abarth@chromium.org changed reviewers:
+ abarth@chromium.org

[abarth-chromium, 2015-01-14 00:01:00]

lgtm

[esprehn, 2015-01-14 00:01:33]

Committed patchset #1 (id:1) manually as
e8fec7dafc9a5226234940697e6ced5bebeaf35d (presubmit successful).