Intent to unship: canvas 2D context mozDash and mozDashOffset.

[Thomas Wisniewski]

The standard versions of these features have been in Firefox since
version 27, the MDN pages have noted that they are deprecated since at
least the middle of last year, and no addons on DXR show that they're
used (except as fallbacks if the standard variant is missing). Other
browsers have also supported the standard versions for at least as
long.

Bug for removal: https://bugzilla.mozilla.org/show_bug.cgi?id=931389
Deprecation notice:
https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/setLineDash
(in the Gecko-specific notes)
Link to standard:
https://html.spec.whatwg.org/multipage/scripting.html#dom-context-2d-setlinedash