Add two way data binding.

Add two way data binding.

All reflected attributes two way bind on SkyElement, so now doing
<sky-element name="sky-input" attributes="value:string"> is enough
to get two way binding on the value attribute so users doing
<sky-input value="{{ inputValue }}"> will get the inputValue property
updated as the user types.

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

Committed: https://chromium.googlesource.com/external/mojo/+/04f96a9e12334f589b13b8b869a495e7e11ad96c

[abarth-chromium, 2015-01-15 22:15:10]

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

[abarth-chromium, 2015-01-15 22:15:11]

lgtm

[esprehn, 2015-01-15 23:03:49]

Committed patchset #2 (id:20001) manually as
04f96a9e12334f589b13b8b869a495e7e11ad96c (presubmit successful).