Investigate extended lifetime shared workers for sending URL Metric data

[westonruter]

Feature Description

There is a new extended lifetime shared workers currently in origin trial in Chrome. It allows for logic to run after a page has unloaded, for up to 30 seconds. This would be an alternative to the current approach implemented in #1893 via #1959 which keeps re-compressing URL Metric data whenever it is changed so that at the moment the page is closed, the pagehide event will be able to send the data without needing to invoke an asynchronous API to compress the data, which then often prevents the data from ever being sent in the beacon.

Related:

• Explore use of the fetchLater API should be used instead of using keepalive fetch at pagehide when available #1970

[b1ink0]

After investigating extended lifetime shared workers, I've created a proof of concept that integrates this new API for submitting URL Metric data, and it seems to be working well in practice.

Here's my proof of concept (POC):
b1ink0@21456ef

In the POC, compression is handled by a shared worker so it does solve the issue of data not being sent.

One thing I discovered is we can't seem to reliably detect if shared worker has the extendedLifetime property. From my research in the POC, I've added browser version detection, but as this feature is still in origin trial there's a need to turn on a flag - we can't reliably detect the support until it's enabled by default in future versions.

In this POC, URL metrics are sent if the browser tab becomes hidden, but with help of shared workers we can send metrics when the tab is actually closed. However, this could bring issues of multiple same URL tabs opened, causing multiple tabs trying to queue metrics for the same URL. I was thinking of introducing a unique ID for each tab, but then it will send metrics for the same URL multiple times.

Unrelated question, but what percentage does Can I Use show for a feature like fetchLater being usable? It's currently at 64%, and Firefox still doesn't support it.

cc: @westonruter