Let gyp create ninja files by default on linux.

Let gyp create ninja files by default on linux.

If a generator is set explicitly (via GYP_GENERATORS or via -f or via
chromium.gyp_env), it will have precedence over the new default.

If you've used make until now, run `ninja -C out/Debug` instead of `make` to
build.

If you're using goma, go/ma has documentation on how to use goma with ninja.
(It's the same as make, except that CC / CXX are now picked up at `gclient sync`
/ `gclient runhooks` / `build/gyp_chromium` time instead of at build time.)

If you can't use ninja for some reason, `export GYP_GENERATORS=make` and sync
again. Please also send me (thakis@chromium.org) an email explaining why ninja
does not work for you.

This will also switch all bots that don't explicitly set a build tool (including
the public bots). compile.py will use ninja instead or make based on if
build.ninja or Makefile are newer, so they should build the right thing
automatically. And since built products end up in the same place, packaging
should do the right thing too.

BUG=239257
R=mark@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=199603

[Mark Mentovai, 2013-05-10 22:31:28]

LGTM

[commit-bot: I haz the power, 2013-05-11 03:03:59]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/thakis@chromium.org/15100009/1

[Nico, 2013-05-11 03:20:42]

Committed patchset #1 manually as r199603.