Bug 283448 – [mp4] requestVideoFrameCallback returns for the wrong frame

• Status: NEW
• Component: Media

[Jean-Yves Avenard [:jya]]

In bug 

the test does:
```
        v.src = "content/test-video-only.mp4";
        await waitFor(v, 'loadedmetadata', true);
        // time of the last video frame.
        v.currentTime = 6.009;

        const seekPromise = waitFor(v, 'seeked', true);
        const data = await waitForVideoFrame(v);
        testExpected(data[1].mediaTime, 6.009);
        testExpected(data[1].width, 320);
        testExpected(data[1].height, 240);
```

the time of the last frame is 6.009 to 06.042

the test fails with:
```
--- /Users/jyavenard/Work/webkit/OpenSource/WebKitBuild/Debug/layout-test-results/media/media-rvfc-paused-offscreen-mp4-expected.txt
+++ /Users/jyavenard/Work/webkit/OpenSource/WebKitBuild/Debug/layout-test-results/media/media-rvfc-paused-offscreen-mp4-actual.txt
@@ -1,4 +1,4 @@
-EXPECTED (6.009 == '6.009') OK
+EXPECTED (5.976 == '6.009'), OBSERVED '5.976' FAIL
 EXPECTED (320 == '320') OK
 EXPECTED (240 == '240') OK
```

5.976 is the time of the second last frame.

Per spec:
```
mediaTime, of type double
The media presentation timestamp (PTS) in seconds of the frame presented (e.g. its timestamp on the video.currentTime timeline). MAY have a zero value for live-streams or WebRTC applications.
```
https://wicg.github.io/video-rvfc/#video-frame-callback-metadata-attributes

currentTime is 6.009 as this is where we seeked to.

[Radar WebKit Bug Importer]

<rdar://problem/140638350>