DEPS.xwalk: Bring in v8-crosswalk.

Start tracking v8-crosswalk in src/v8. v8-crosswalk is a V8 fork along the
lines of chromium-crosswalk and blink-crosswalk. For now its main purpose is
to have all the SIMD.js optimizations some folks have been working on. Other
than that, it should be the usual V8 we all know.
Merge pull request #1699 from rakuco/pull-v8-crosswalk

DEPS.xwalk: Bring in v8-crosswalk.
[Runtime] Add manifest handler for widget packaged applications.

The CL parses manifest keys required by JavaScript widget interface,
and save mapped key value pairs in ManifestData, so that it can be read
by widget extension.

We can also extend this CL to validate the widget configuration keys in
future.
[Tizen][Runtime] Implement Widget API.

This commit supports Widget API on Tizen(exclude 'preferences' attribute).
The API provides application programming interfaces that enables ability
to access some of the metadata declared in a widget's configuration
document, such as 'name', 'author', 'description'.

Spec:
http://www.w3.org/TR/widgets-apis/#the-widget-interface

The 'preferences' attribute implementation will be provided later.
Add widget preference implementation.