[NTP] Fix flaky LocalNTPCustomLinksTests

Remove the Most Visited 'loaded' event listener after the 'loaded'
message is sent. This was interfering with other domAutomationController
listeners in subsequent script executions.

Bug: 903265
Change-Id: Iccd33d3eb04330a23880735b48a00d2f97a50866
Reviewed-on: https://chromium-review.googlesource.com/c/1327425
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606920}

chrome/browser/ui/search/local_ntp_test_utils.cc

@@ -66,10 +66,11 @@ void NavigateToNTPAndWaitUntilLoaded(Browser* browser, int delay) {
         if (tilesAreLoaded) {
           return true;
         }
-        window.addEventListener('message', function(event) {
+        let loaded = window.addEventListener('message', function(event) {
           if (event.data.cmd == 'loaded') {
+            window.removeEventListener('message', loaded);
             setTimeout(() => {
-             domAutomationController.send('NavigateToNTPAndWaitUntilLoaded');
+              domAutomationController.send('NavigateToNTPAndWaitUntilLoaded');
             }, %d);
           }
         });