ninja: Change the default target from 'build everything' to 'build target "all"'.
ninja: Change the default target from 'build everything' to 'build target "all"'.

'all' are all targets transitively referenced by all.gyp.  For example:

all.gyp:
  targets: [
    {
      'target_name': 'main_target',
      'dependencies': [
        'dep.gyp:target1',
      ],
    },
  ]

dep.gyp:
  targets: [
    { 'target_name': 'target1' },
    { 'target_name': 'target2' },
  ]

Here, 'all' does include target1 from dep.gyp, but not target2.
The generator still writes a ninja file for target2, and it can
be built with 'ninja target2', but just 'ninja' will no longer
build it.

BUG=127952
TEST=Running just `ninja -C out/Release` doesn't produce errors.
Review URL: https://chromiumcodereview.appspot.com/10399055

csulmone/gyp,csulmone/gyp,csulmone/gyp,csulmone/gyp