[iOS][test runner] Use warning log level in requests.

When reporting test results to ResultSink, this could reduce log spams
stating post success for each test result.

Bug: 1157537
Change-Id: Ibdb67d22e192c1967038a4ceaeef7d2d82f0edd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2590647Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841667}

ios/build/bots/scripts/result_sink_util.py

@@ -106,11 +106,15 @@ class ResultSinkClient(object):
       # Ensure session is closed at exit.
       atexit.register(self.close)
 
+    logging.getLogger("requests").setLevel(logging.WARNING)
+
   def close(self):
     """Closes the connection to result sink server."""
     if not self.sink:
       return
     LOGGER.info('Closing connection with result sink server.')
+    # Reset to default logging level of test runner scripts.
+    logging.getLogger("requests").setLevel(logging.DEBUG)
     self._session.close()
 
   def post(self, test_result):