Extend timeout for poster display.

Debug and slow bots can exceed the 1s timeout here.  Unfortunately
there's no way to tell if this has been displayed yet, so the best
we can do is add a longer timeout.

BUG=513398
TEST=bots

Review URL: https://codereview.chromium.org/1296343003

git-svn-id: svn://svn.chromium.org/blink/trunk@200900 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/LayoutTests/media/video-poster-after-loadedmetadata.html

@@ -9,7 +9,7 @@ function startTest() {
   video.addEventListener("loadedmetadata", function () {
     video.poster = "content/abe.png";
     if (window.testRunner)
-      setTimeout(function() { testRunner.notifyDone(); }, 1000);
+      setTimeout(function() { testRunner.notifyDone(); }, 2000);
   });
 
   video.src = "resources/test-positive-start-time.webm";