Intent to ship: enterkeyhint attribute

[Makoto Kato]

enterkeyhint is a HTML element attribute that is for virtual keyboard
layout. Virtual keyboard can change "ENTER" key cap to others such as
"GO", "SEND" and etc.

Now WebKit and Blink (all platforms) support this attribute and
getter/setter API.

This was implemented by bug 1490661, then I would like to ship this on
all channels.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1648332

Link to standard:
https://html.spec.whatwg.org/dev/interaction.html#input-modalities:-the-enterkeyhint-attribute

Platform coverage: All. Desktop version is unaffected to change this
since the OS API for virtual keyboard doesn't have this feature.

Preference: This is enabled/disabled via dom.forms.enterkeyhint

Devtools support: N/A. DevTools doesn't have software keyboard emulation.

Other browsers: Both Chrome and Safari support this.

web-platform-tests: reflection test for HTMLElement is
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/html/dom/reflection-misc.html.

Restricted to secure contexts: No