mb: Allow overriding the path to the gyp script

tools/mb/mb.py

Index: tools/mb/mb.py
diff --git a/tools/mb/mb.py b/tools/mb/mb.py
index 2c16266c641b09fcb0b12c41863af082a377914f..5421e231284085e4227ab82a3fc2fc99d51f30b4 100755
--- a/tools/mb/mb.py
+++ b/tools/mb/mb.py
@@ -84,6 +84,9 @@ class MetaBuildWrapper(object):
                             '(default is //tools/mb/mb_config.pyl)')
       subp.add_argument('-g', '--goma-dir',
                         help='path to goma directory')
+      subp.add_argument('--gyp-script',

[Michael Achenbach, 2016/06/03 07:36:38]

FYI: I didn't specify a default here to keep the tests happy. PathJoin is not
mocked yet when this is called.

[Dirk Pranke, 2016/06/03 17:59:24]

I don't quite understand this comment; PathJoin() should already be defined on
the object (both here and in the subclass)? 

However, alternatively, you could define a .default_gyp_script in the __init__()
method just like there's a default_config.

It's not all that important either way, though.

[Michael Achenbach, 2016/06/03 19:55:03]

That's what I tried. PathJoin was not defined in the init method. And setting
default_gyp_script with path.join there made the test fail on windows.

Probably it's defined in ParseArgs. I could just call it there to calculate the
default on the fly without setting it in a field.

+                        help='path to gyp script relative to project root '
+                             '(default is build/gyp_chromium)')
       subp.add_argument('--android-version-code',
                         help='Sets GN arg android_default_version_code and '
                              'GYP_DEFINE app_manifest_version_code')
@@ -1105,7 +1108,7 @@ class MetaBuildWrapper(object):
 
     cmd = [
         self.executable,
-        self.PathJoin('build', 'gyp_chromium'),
+        self.args.gyp_script or self.PathJoin('build', 'gyp_chromium'),
         '-G',
         'output_dir=' + output_dir,
     ]