Let test driver export json results.

tools/run-tests.py

Index: tools/run-tests.py
diff --git a/tools/run-tests.py b/tools/run-tests.py
index cfca80f64b7e629062016df38ecab28ada79dc18..61f91f0a5e3a5a1a7cefe68d8f661a3ac6c2e16c 100755
--- a/tools/run-tests.py
+++ b/tools/run-tests.py
@@ -173,6 +173,8 @@ def BuildOptions():
                     help=("Quick check mode (skip slow/flaky tests)"))
   result.add_option("--report", help="Print a summary of the tests to be run",
                     default=False, action="store_true")
+  result.add_option("--json-test-results",
+                    help="Path to a file for storing json results.")
   result.add_option("--shard-count",
                     help="Split testsuites into this number of shards",
                     default=1, type="int")
@@ -463,6 +465,9 @@ def Execute(arch, mode, args, options, suites, workspace):
   if options.junitout:
     progress_indicator = progress.JUnitTestProgressIndicator(
         progress_indicator, options.junitout, options.junittestsuite)
+  if options.json_test_results:
+    progress_indicator = progress.JsonTestProgressIndicator(
+        progress_indicator, options.json_test_results, arch, mode)
 
   run_networked = not options.no_network
   if not run_networked: