[Tools] Groundwork for tracking Chromium's dependencies with git.

This patch series does the majority of the work necessary for actually
making the full switch to git in a way that does not break anyone's
workflow and allows the bots to switch back and forth between git and
SVN while this is being reviewed. It takes a much less radical approach
than #2109 for that matter.

The main idea here is to move most of the logic in
`generate_gclient-xwalk.py` into `DEPS.xwalk` itself. Quoting the last
and most important commit in this series:

> Most of the code in `generate_gclient-xwalk.py` should not exist: it
> functioned mostly as a translator that read values from `DEPS.xwalk`
> and changed them into a format suitable for gclient.
>
> We can get rid of this intermediate step by writing `DEPS.xwalk` in a
> gclient-compatible format. In practice, this means that `DEPS.xwalk`
> is now responsible for:
>
> * Setting git revision values for repositories we track, such as
>  chromium-crosswalk and ozone-wayland (as before).
>
> * Automatically ignoring some directories in the checkout.
>
> * Setting variables parsed by gclient, such as `solutions`.
>
> All this also allows us to remove a lot of validation code that was
> scattered around `generate_gclient-xwalk.py`.
>
> `generate_gclient-xwalk.py` now exists merely for compatibility, as it
> still needs to parse the `XWALK_OS_ANDROID` environment variable and
> the `--cache-dir` option and append a few more lines to
> `.gclient-xwalk` accordingly. I can see these features being moved
> somewhere else and the script getting removed altogether in the
> future.

The other commits are just small simplifications to the code to get rid
of features that were never used. The most significant of them is
_Ignore a smaller subset of directories in a checkout_, which does end
up checking out a few additional smaller repositories.

With this pull request, most of the work in XWALK-1894 is done, even
though there is no visible difference to any users yet and all
repositories still come from SVN just like before.
Isn't this still using svn for checkout?