un-revert "add a ReportBuildStartStage" and "clean up UploadMetadata"

This change re-lands https://chromium-review.googlesource.com/195494 and
https://chromium-review.googlesource.com/195495 , which were reverted
due to crbug.com/366333

CQ-DEPEND=CL:196892
BUG=chromium:362212
TEST=Tested when first landed, + see tests for depended CL.

Change-Id: Ibdf5a50b8280c9f236a3d18d9502530c45effc9d
Reviewed-on: https://chromium-review.googlesource.com/196788
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 9494dee..025d6c7 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -363,6 +363,7 @@
         print_report = False
         success = self._ReExecuteInBuildroot(sync_instance)
       else:
+        self._RunStage(stages.ReportBuildStartStage)
         self.RunStages()
 
     except Exception as ex: