[Tools] Remove support for custom variables in DEPS.xwalk.

We have not needed this for several releases, so get rid of the code to
simplify generate_gclient-xwalk.py a bit.

This is part of the preparations for XWALK-1894.
[Tools] Ignore a smaller subset of directories in a checkout.

The XWALK_SYNC_IGNORE environment variable was never used, and the
upstream wiki page the function
mentions (http://dev.chromium.org/developers/how-tos/get-the-code) does
not contain the "Reducing the size of your checkout" section anymore.

Simplify _AddIgnorepathFromEnv() by just ignoring directories outside
src/ that are never used in a regular Crosswalk checkout.

This is another commit doing some groundwork for XWALK-1894.
[Tools] Stop allowing different DEPS files in generate_gclient-xwalk.py.

This feature has never been used, so let us settle on DEPS.xwalk and
simplify the code a bit.

Also related to XWALK-1894.
[Tools] Remove pylint workaround in generate_gclient-xwalk.py.
[Tools] Move most of the generate_gclient-xwalk's logic to DEPS.xwalk.

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.

This is a significant step in the preparations for XWALK-1894.
Merge pull request #2135 from rakuco/preparation-for-xwalk-1894

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