Fix mb.py string GN args

Fixes a recent regression where mb.py started incorrectly classifying
tests due to a change in how GN args were parsed from the build
directory. This re-adds quotes around string args, which brings the
previous behavior back.

Bug: 1159618
Change-Id: Ide52674e258d2a2698d0011928bd0d452a92cb8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596640
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838252}

tools/mb/mb.py

@@ -875,6 +875,11 @@ class MetaBuildWrapper(object):
 
     args_contents = ReplaceImports(args_contents)
     args_dict = gn_helpers.FromGNArgs(args_contents)
+    # Re-add the quotes around strings so they show up as they would in the
+    # args.gn file.
+    for k, v in args_dict.items():
+      if isinstance(v, str):
+        args_dict[k] = '"%s"' % v
     return ' '.join(['%s=%s' % (k, v) for (k, v) in args_dict.items()])
 
   def Lookup(self):