Follow up from bug 1713715.

Instead we zoom to the containing iframe.

Chrome and Safari do the same currently.

Doesn't need to block Fission MVP because Chrome and Safari have the same behavior.

As previously mentioned Chrome on macOS does not implement this, it just zooms to the boundary of any remote iframe. Surprisingly Chrome on Android does seem to implement this. Testing it is a little bit tricky though as only logged in sites get process separation by default on Chrome on Android. Based on

https://security.googleblog.com/2021/07/protecting-more-with-site-isolation.html

one can make all sites get their own process by flipping the flag at chrome://flags/#enable-site-per-process and then double check to make sure you get the process structure you expect by looking at chrome://process-internals

Though there's no open bugs corresponding to these test cases, there were no
test cases to make sure touch-action:manipulation prevents double-tap-to-zoom
on touchscreen and doesn't prevent it on touchpad.

I've confirmed both Chrome and Safari have the same behavior on touchpad.

Depends on D186321

We will use the function for double-tap-to-zoom calculations on the main-thread.

Depends on D186322

Depends on D186323

Depends on D186324

Depends on D186325