Report what changes were in a given CQ run in metadata.json.

It is helpful to know what changes are included in a CQ run
for the purposes of gathering stats.

I have added data that looks like this:

"changes": [{"gerrit_number": "1234", "patch_number": "1", "internal": false}]

BUG=chromium:270774
TEST=New unit test. mario-paladin trybot run.

Change-Id: I97816afed03eb50d4bb2abeca3c779b9c0e90d17
Reviewed-on: https://gerrit.chromium.org/gerrit/65805
Commit-Queue: David James <davidjames@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: David James <davidjames@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 4a54d1e..8c9f384 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -297,6 +297,7 @@
     print_report = True
     exception_thrown = False
     success = True
+    sync_instance = None
     try:
       self.Initialize()
       sync_instance = self.GetSyncInstance()
@@ -328,7 +329,7 @@
       if print_report:
         results_lib.WriteCheckpoint(self.options.buildroot)
         self._RunStage(stages.ReportStage, self.archive_stages,
-                       self.release_tag)
+                       self.release_tag, sync_instance)
         success = results_lib.Results.BuildSucceededSoFar()
         if exception_thrown and success:
           success = False