Add more fields to try-results --json for WPT importer

Context: The WPT importer consumes the JSON produced by
git cl try-results is consumed and uses it to determine
when the CQ has passed (whether all non-experimental cq
jobs were successful) and where layout test results are
stored.

Added fields:
  created_ts: This could be used to sort builds by time.
  tags: Includes user_agent tag and experimental tag, as well as
  experimental: whether the job is experimental

Bug: 792652
Change-Id: Ic6099a21b21db668b88b1a8e54aefd84529a2a91
Reviewed-on: https://chromium-review.googlesource.com/819573
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
diff --git a/git_cl.py b/git_cl.py
index 171e2fd..e7d5c26 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -716,14 +716,18 @@
   """
 
   def convert_build_dict(build):
+    """Extracts some of the information from one build dict."""
+    parameters = json.loads(build.get('parameters_json', '{}')) or {}
     return {
         'buildbucket_id': build.get('id'),
-        'status': build.get('status'),
-        'result': build.get('result'),
         'bucket': build.get('bucket'),
-        'builder_name': json.loads(
-            build.get('parameters_json', '{}')).get('builder_name'),
+        'builder_name': parameters.get('builder_name'),
+        'created_ts': build.get('created_ts'),
+        'experimental': build.get('experimental'),
         'failure_reason': build.get('failure_reason'),
+        'result': build.get('result'),
+        'status': build.get('status'),
+        'tags': build.get('tags'),
         'url': build.get('url'),
     }