Test polling behaviour of ConnectionHealthChecker::VerifySentData

VerifySentData polls /proc/tcpinfo to verify data sent over a given
connection is actually delivered. Add unit-test where it polls twice.

BUG=chromium:234844
TEST=build and run unit-tests

Change-Id: I09c86f5771089521670a5ffbb39d2a3162c4fe3e
Reviewed-on: https://gerrit.chromium.org/gerrit/50544
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
diff --git a/connection_health_checker.h b/connection_health_checker.h
index 1c95ba9..936cc2c 100644
--- a/connection_health_checker.h
+++ b/connection_health_checker.h
@@ -220,6 +220,9 @@
   // Number of successful send attempts currently active health check.
   short num_successful_sends_;
 
+  // Snooze time while polling for updated /proc/tcpinfo
+  int tcp_state_update_wait_milliseconds_;
+
   // Temporarily store the result of health check so that |report_result_|
   // can report it.
   Result health_check_result_;