[PaintTiming] Let videos trigger FCP

This CL adds a call to SetImagePainted() so that FCP fires when there we
paint a video.

Bug: 1071450
Change-Id: I9590517fd3355226096fd62079a3a4aa6eb50735
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276244
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784189}

paint-timing/fcp-only/fcp-video-frame.html

@@ -6,6 +6,7 @@
 <script src="../resources/utils.js"></script>
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
+<script src="/common/media.js"></script>
 <video id="video" autoplay></video>
 <script>
   promise_test(async t => {
@@ -14,7 +15,7 @@
       await assertNoFirstContentfulPaint(t);
       // Set actual video content to trigger FCP.
       const video = document.getElementById('video');
-      video.setAttribute('src', '/media/test.mp4');
+      video.src = getVideoURI('/media/test');
       await new Promise(resolve => {
         video.oncanplay = resolve;
       });