mb.py: Ignore commented-out gn args

Otherwise, a line containing '#is_asan=true' can add run_with_asan to
the beginning of the command, which will fail.

R=brucedawson@chromium.org, thakis@chromium.org

Change-Id: I31cf9c35cfc0ca04f4c6cc3b5f2376b0c0c58f3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033630Reviewed-by: Reid Kleckner <rnk@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738041}

tools/mb/mb.py

@@ -821,6 +821,9 @@ class MetaBuildWrapper(object):
       args_contents = self.ReadFile(gn_args_path)
     gn_args = []
     for l in args_contents.splitlines():
+      l = l.split('#', 2)[0].strip()
+      if not l:
+        continue
       fields = l.split(' ')
       name = fields[0]
       val = ' '.join(fields[2:])