Intent to implement and ship: CSS property touch-action:pinch-zoom

[Kartikaya Gupta]

We intend to enable the following feature in m-c soon, and ship it in Gecko
85.

Summary:
Support for the "pinch-zoom" keyword value for the touch-action CSS
property

Bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1329241

Standard:
https://compat.spec.whatwg.org/#touch-action

Platform coverage:
All (desktop and Android)

Preference:
touch-action is already behind the layout.css.touch_action.enabled pref
(enabled by default) and this new value will be covered by the same pref.

DevTools bug:
none, I don't believe this requires any specific devtools work

Other browsers:
According to
https://caniuse.com/mdn-css_properties_touch-action_pinch-zoom Chrome has
supported this since v56 (~Jan 2017); IE/Edge have supported it since Oct
2013 or inception; Safari has supported it since v13 (~Sep 2019). We're the
only major browser that doesn't support it yet.

web-platform-tests:
Covered by web-platform/tests/pointerevents/compat/
Additionally tested by some Gecko-specific mochitests that exercise the
Gecko compositor hit-testing codepath.