[Media Router] Fix MediaRouterIntegrationIncognitoBrowserTest.ReconnectSession

event_page_request_manager has "TerminateRoute()" in pending_requests_. When
clearing pending_requests_ and destroying "TerminateRoute()" callback, the
binded mojo callback (parameter) is destroyed but never invoked. Mojo has some
check which would crash if mojo callback is never invoked.

"TerminateRout()" is added to pending_requests_ because
MediaRouterBase::OnIncognitoProfileShutdown() adds it after test teardown
unregisters MR extension.

Fix it by explicitly terminate session in ReconnectSession test.

Bug: 818766
Change-Id: I02d0019ba52d3d9213ba6edec34006a7ffac91c0
Reviewed-on: https://chromium-review.googlesource.com/952292
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Derek Cheng <imcheng@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541605}

chrome/test/media_router/media_router_integration_browsertest.cc

@@ -679,6 +679,8 @@ void MediaRouterIntegrationBrowserTest::RunReconnectSessionTest() {
       "window.domAutomationController.send(reconnectedSession.id)",
       &reconnected_session_id));
   ASSERT_EQ(session_id, reconnected_session_id);
+
+  ExecuteJavaScriptAPI(web_contents, kTerminateSessionScript);
 }
 
 void MediaRouterIntegrationBrowserTest::RunReconnectSessionSameTabTest() {