Combine element registries from sky-binder and sky-element.

Combine element registries from sky-binder and sky-element.

We now have an element-registry.sky module that exposes API to register
elements with attributes and event handlers. Right now it's an API on
the module itself, eventually the module will export an ElementRegistry
object since we'll need one per module once the custom element registry
is one per module.

R=abarth@chromium.org, ojan@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/956a552a2fb4e470003bd75ffb381fc42d30cac1

[abarth-chromium, 2015-01-16 02:50:27]

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

[abarth-chromium, 2015-01-16 02:50:27]

lgtm

[esprehn, 2015-01-16 04:31:37]

Committed patchset #1 (id:1) manually as
956a552a2fb4e470003bd75ffb381fc42d30cac1 (presubmit successful).