[Tools] Use .gclient's value of |cache_dir| in .gclient-xwalk.

This change does not have a big visible effect to most, as it was
introduced mainly for the canary/beta builders and was never documented.

The previous approach was not easy to use at all, as one had to manually
call `gclient sync` with `--nohooks`, call `generate_gclient-xwalk.py`
manually with `--cache-dir` and then run the other hooks.

Instead, just use the same value as `.gclient` so that all repositories
are cached in the same location, thus also reducing the differences
between `.gclient` and `.gclient-xwalk`.

`--cache-dir` is still accepted and used if passed for compatibility
reasons; support for it can be removed altogether in the next major
version (Crosswalk 8).

Related to XWALK-1184 and XWALK-1894.