Make checkdeps.py correctly signal failure when using --json.

This is a followup to https://codereview.chromium.org/75693002/

TBR=joi
BUG=317931

Review URL: https://codereview.chromium.org/75793003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235865 0039d316-1c4b-4281-b951-d872f2087c98

tools/checkdeps/results.py

@@ -126,6 +126,7 @@ class JSONResultsFormatter(ResultsFormatter):
     with open(self.output_path, 'w') as f:
       f.write(json.dumps(self.results))
 
+    return self.results
 
   def PrintResults(self):
     if self.wrapped_formatter: