[Crosswalk] Move away from sync_repos.sh.

[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.

Change-Id: Ifeb4060ea0152d8c373235003a4a5020b0b17fcc
Signed-off-by: Piotr Tworek <p.tworek@samsung.com>