chromium: Switch build system to ninja.

Wanted to do this a long time ago, but never had a reason to do it. But with
Chromium 29 having no make target for chrome_sandbox, we now use ninja as well
as the official build and most other distributions.

The whole build/make flags cruft is now integrated into one buildPhase override
and we just call ninja there by exporting the specific variables.

And this also makes enableParallelBuilding obsolete, as we use NIX_BUILD_CORES
directly now.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>