Chrome extension to capture video of last few minutes of active tab - google-chrome-extension

Question

I am trying to write a Chrome plugin to capture a video of the active tab. My code is based on this post.
When my page action is invoked, I start the recording:
var recordedChunks = null;
var captureOptions = { audio : false, video : true };
chrome.tabCapture.capture(captureOptions,
function(stream) {
if (stream) {
recordedChunks = [];
var options = {mimeType: "video/webm"};
mediaRecorder = new MediaRecorder(stream, options);
mediaRecorder.start();
mediaRecorder.ondataavailable = function(event) {
if (event.data.size > 0) {
recordedChunks.push(event.data);
}
}
}
}
);
When the page action is invoked again, I stop the recording and download a file as:
mediaRecorder.stop();
var blob = new Blob(recordedChunks, {
type: 'video/webm'
});
var url = URL.createObjectURL(blob);
var a = document.createElement('a');
document.body.appendChild(a);
a.style = 'display: none';
a.href = url;
a.download = 'test.webm';
a.click();
window.URL.revokeObjectURL(url);
stream.getVideoTracks()[0].stop();
This works great - I am able to play the downloaded test.webm video.
But I only want to record the last few minutes of video of the active tab. I do not want the recordedChunks array to grow unbounded. So, I tried something like this in the start recording action:
chrome.tabCapture.capture(captureOptions,
function(stream) {
// ...
mediaRecorder.ondataavailable = function(event) {
if (event.data.size > 0) {
recordedChunks.push(event.data);
// CHANGE HERE: keep only the last 1000 blobs
while (recordedChunks.length > 1000) {
recordedChunks.shift();
}
}
}
}
);
But with this modification, the download test.webm video is not playable. How do I capture just the tail of the blob output from MediaRecorder?