mb fixes: fix uninitialized return value on empty file list

and add bot entries for the chromium.webkit GN bots.

[email protected]
BUG=

Review URL: https://codereview.chromium.org/1082393002

Cr-Commit-Position: refs/heads/master@{#325146}
diff --git a/tools/mb/mb.py b/tools/mb/mb.py
index 2a9941b..0834d55 100755
--- a/tools/mb/mb.py
+++ b/tools/mb/mb.py
@@ -417,6 +417,7 @@
       return 0
 
     all_needed_targets = set()
+    ret = 0
     for f in inp['files']:
       cmd = self.GNCmd('refs', self.args.path[0]) + [
              '//' + f, '--type=executable', '--all', '--as=output']