[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.