[Crosswalk] Move away from sync_repos.sh.

Patch implements a fully gclient based featch solution,
moving away from sync_repos.sh completely.

The following actions are done (README.md updated):
- The custom_hook associated to sync_repos.sh in .gclient is removed
- Chromium and WebKit are pulled directly from EFL forks via .gclient-xwalk.
- Crosswalk is pulled as a separate "solition" in .gclient.
- The needed custom hooks are overriden.
[Crosswalk] Move away from custom_hooks needs in .gclient.

Patch adds a DEPS file in tizen_src where 'xwalk' is specified
as a hard dependency. It allows us to greatly simplify
our suggested .gclient.

Our 'custom_hooks' also got to be declared as 'hooks' in DEPS.
[Crosswalk] Updated README.md