Update mb.py to support gn directories with no args.gn

tools/mb/mb.py

Index: tools/mb/mb.py
diff --git a/tools/mb/mb.py b/tools/mb/mb.py
index 2c16266c641b09fcb0b12c41863af082a377914f..2de1d9b6f6a23c7f65c6d98a9c49f94a3dc3daeb 100755
--- a/tools/mb/mb.py
+++ b/tools/mb/mb.py
@@ -528,9 +528,11 @@ class MetaBuildWrapper(object):
     if not self.Exists(mb_type_path):
       gn_args_path = self.PathJoin(self.ToAbsPath(build_dir), 'args.gn')
       if not self.Exists(gn_args_path):
-        self.Print('Must either specify a path to an existing GN build dir '
-                   'or pass in a -m/-b pair or a -c flag to specify the '
-                   'configuration')
+        self.Print('Couldn\'t find %s. Must either specify a path to an '
+                   'existing GN build dir or pass in a -m/-b pair or a -c flag '
+                   'to specify the configuration. If %s is a GN build dir then '
+                   'you can use gn args %s to generate args.gn' %
+                   (gn_args_path, build_dir, build_dir))

[Dirk Pranke, 2016/05/31 20:39:03]

Ah, good catch. 

However, the MB code is actually wrong since, as you've discovered, you can have
a perfectly legit gn-built build directory w/o any args.gn file.

MB needs to be smarter about this and check for the existence of a
toolchain.ninja file in that directory as well, as we do in 

https://code.google.com/p/chromium/codesearch?q=factory.py#chromium/src/third...

MB's commands also work w/ GYP builds, so this message isn't quite right as-is
and we might want to use a slightly different message like the one on line 87 in
the above file.

Do you want to make that change, or shall I?

[brucedawson, 2016/05/31 20:44:03]

Making "it just work" would be better. Is checking for toolchain.ninja the
appropriate thing to do?

The message could also be improved, perhaps, but if the check is accurate then
that probably isn't necessary.

If the change is just to check for toolchain.ninja instead of args.gn then I can
do that.

[Dirk Pranke, 2016/05/31 20:48:23]

It's a little bit more complicated. If we think this is a GN build directory,
then we try to read
from args.gn to figure out the list of args (see line 543). So you'd need to
modify GNValsFromDir() to handle a missing
args.gn file.

But that should still be straightforward. 

And, yes, checking for toolchain.ninja is sufficient as long as you're not
trying to defend against the case where you have GYP-generated and GN-generated
files in the same directory (which I think we assert for and error out on
somewhere anyway).

         return {}
       else:
         mb_type = 'gn'