Intent to enable: touch-action (on Nightly)

[Kartikaya Gupta]

In the next few days, I intend to enable support for touch-action on
Nightly (#ifdef NIGHTLY_BUILD), for all platforms. The implementation
is behind the layout.css.touch_action.enabled property. touch-action
is spec'd as part of the Pointer Events spec [1] - the rest of the
spec is currently being implemented in Firefox, but is not yet ready
to be enabled. The touch-action CSS property is relatively independent
of the rest, and can be enabled separately. The patch to turn it on is
in bug 1029631, and the dependency tree has various implementation
bugs if you go deep enough.

Note that turning on touch-action will be detectable by web content on
all platforms (e.g. by checking for the existence of
getComputedStyle(document.body).touchAction). However, the
implementation only does something on platforms/configurations where
we have touch event support. Those platforms are: (1) Android, (2)
Linux with e10s and MOZ_USE_XINPUT2=1 in the environment, (3) Windows
Nightly with e10s enabled [2].

Other UAs have already mostly implemented and shipped touch-action
support [3]. I don't expect much in the way of interop or webcompat
issues with this change, but please file bugs if you notice something
that might be caused by this. Later this month, I will be meeting with
other browser vendors and hammer out any issues that come up, and I
expect to let this ride the trains in Fx 50 or (more likely) 51, along
with general touch event support on Windows.

Cheers,
kats

[1] https://www.w3.org/TR/pointerevents/#declaring-candidate-regions-for-default-touch-behaviors
[2] Since touch-enabled Windows devices trigger accessibility
activation, they run with e10s disabled by default, even on Nightly.
You have to set browser.tabs.remote.force-enable=true to actually get
e10s on these devices. The plan is to make e10s accessibility-friendly
in 51 or 52 at which point this caveat goes away.
[3] http://caniuse.com/#search=touch-action