Update test event-source-constructor.html

This CL update the blink layout test:
fast/eventsource/eventsource-constructor.html

Depending on timing, the third EventSource:
~~~
   new EventSource("http://disallowed.example.com/");
~~~
may start before the test finishes. It results in a new console error
message displayed.

This CL prevent the console error message to be displayed.

Note: I am working on https://crbug.com/831155. It causes the frame to commit
a navigation slightly faster. As a result, some tests that are relying on
how things are scheduled in blink are failing.

Bug: 831155
Change-Id: I8a6bf0c1f1203be1e529b7eb6e3192849f7c518d
Reviewed-on: https://chromium-review.googlesource.com/1102324Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567952}

third_party/WebKit/LayoutTests/fast/eventsource/eventsource-constructor.html

@@ -11,6 +11,11 @@ function log(msg) {
 }
 
 if (window.testRunner) {
+    // Depending on timing, this test may end before or after a CSP event source
+    // violation is fired. Make this test deterministic by don't dumping console
+    // messages.
+    testRunner.setDumpConsoleMessages(false);
+
     testRunner.dumpAsText();
     testRunner.waitUntilDone();
 }