tools/mb/mb.py should handle gn args with different number of spaces

Currently GNArgsFromDir expects gn arg to have exactly one space around
assignment operator (target_os = "android"). It removes both spaces so
that later the code could to a check in the form
('target_os="android"' in vals['gn_args']). If an arg doesn't have
spaces or have extra (target_os ="android"), the logic works
incorrectly.

This CL fixes GNArgsFromDir to break name/value line on '=' and strip
spaces around both name and value.

BUG=1066289

Change-Id: I958b2cb400415f33f6b5310da0f127df715bd059
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2127742
Commit-Queue: Pavel Yatsuk <pavely@chromium.org>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754718}

tools/mb/mb.py

@@ -884,10 +884,8 @@ class MetaBuildWrapper(object):
       l = l.split('#', 2)[0].strip()
       if not l:
         continue
-      fields = l.split(' ')
-      name = fields[0]
-      val = ' '.join(fields[2:])
-      gn_args.append('%s=%s' % (name, val))
+      (name, value) = l.split('=', 1)
+      gn_args.append('%s=%s' % (name.strip(), value.strip()))
 
     return ' '.join(gn_args)