Fix potential access to deprecated web API's

[Hamish-taylor]

The problem

This is the timing object, and on this line we are accessing .responseEnd which has been deprecated in favor of this. We do actually collect this timing object when we get the performance metrics here, but I don't fully understand what happens with that information.

The result

The result of this is that the events are still sent, but due to missing/bad data the events are never displayed on the frontend.

The fix

• Added checks to make sure we are not accessing the undefined property. <- there's probably a better way of handling this
• Allowed the new timing object to be used

My current questions

• How do we ensure the fix is backwards compatible
  • According to caniuse.com The 'new' metric is supported across 93.12% or users compared to 95.71% with the old method (a difference of 2.59%).
• Do the new timing events measure the same things?
  • From my testing it appears that they do

[darcythomas]

Looks like you have a broken test.

Other than that it looks good :P

[xenolightning]

Great work @Hamish-taylor!

The only comment I had was to maybe bump minor version, but not required