Fix flaky xssAuditor test.

The layout test: object-src-inject.html is flaky.
Sometimes, this line is printed:
```
   CONSOLE MESSAGE: Blink Test Plugin: initializing
```

It is fired from BlinkTestInstance::Init(...);

It is racing with the end of the layout test.
This CL disables console error dump to avoid this issue.

Bug: 862959, 831155
Change-Id: I912e71b9ff8b7b5ccd7a4716f111f2017493e37a
Reviewed-on: https://chromium-review.googlesource.com/1135003Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578994}

third_party/WebKit/LayoutTests/http/tests/security/xssAuditor/object-src-inject.html

@@ -7,6 +7,7 @@ if (window.testRunner) {
   testRunner.dumpAsText();
   testRunner.setXSSAuditorEnabled(true);
   testRunner.waitUntilDone();
+  testRunner.setDumpConsoleMessages(false); // See https://crbug.com/862959
   notifyDoneAfterReceivingLoaded();
 }
 </script>