Update the gyp Linux build:

* Add Linux settings to target_defaults in common.gypi so
  gyp-generated SConscript files no longer depend on
  build/SConscript.main or the Hammer infrastructure.
* Copy the FilterOut() function from Hammer to the chromium_builders.py
  Tool module.
* Add a ChromiumLoadableModule() builder to chromium_builders.py.
* Add dependencies on the 'views' library to the chrome link (target 'app').
* Add missing views/*/*_unittest.cc modules to the 'unit_tests' target.
  Exclude all but the one that builds on Linux from the non-Windows builds.
* Crib a list of chrome/views files to exclude from the Linux build
  from the old SCons configuration.
* Add a new build/linux/system.gyp file with new 'settings' targets
  to encapsulate the pkg-config checks for gtk+-2.0, nss and pangoft2.
* Add depenedencies in the other targets on the new gtk, nss and
  pangoft2 'settings' targets from build/linux/system.gyp.
* Add a pkg_config_wrapper.py script that keeps gyp happy by
  simply exiting 0 if the package isn't found.
* DEPS roll for latest gyp changes to support the above.
Review URL: http://codereview.chromium.org/42340

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@12228 0039d316-1c4b-4281-b951-d872f2087c98