Do not use scripting inside gclient

After recent changes on depot_tools, using "for loops" inside gclient
config file apparently is not possible anymore. Also, the "solutions"
now has to be explicitly declared.

BUG=XWALK-2379

(cherry picked from commit 60aaa8bec57badb4d98e13ed9b36cf425211d990)
DEPS.xwalk: Remove duplicate lines.

Remove a few duplicate comments added by 60aaa8bec ("Do not use
scripting inside gclient").

(cherry picked from commit e208770a0473c7600f24599676856d12f89a00cc)
Merge pull request #2348 from rakuco/backport-gclient-fixes

Backport gclient fixes