[test] Return target name on failures.

This information can be used on the buildbot side to only
rebuild the failing target for bisection.

BUG=chromium:511215
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1313353006

Cr-Commit-Position: refs/heads/master@{#30617}

tools/testrunner/local/progress.py

@@ -375,6 +375,7 @@ class JsonTestProgressIndicator(ProgressIndicator):
       # TODO(machenbach): This stores only the global random seed from the
       # context and not possible overrides when using random-seed stress.
       "random_seed": self.random_seed,
+      "target_name": test.suite.shell(),
       "variant": test.variant,
     })