canvas: Make 2D context state creation failable and use dom_canvas_backend pref for backend selection (#38310)

Before script just crashed in those cases because IPCSender was dropped,
now we send `None` to tell script about the failure and fail getContext
or registerPainter accordingly.
This PR also unifies `dom_canvas_{backends}_enabled` prefs into
`dom_canvas_backend` which is more flexible in multi-backends scenarios.

Reviewable per commit.

Testing: Added servo specific WPT test.

---------

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

tests/wpt/mozilla/meta/MANIFEST.json

@@ -12970,6 +12970,13 @@
        null,
        {}
       ]
+     ],
+     "invalid.html": [
+      "d50ff1030c48794dfc92083b6fecde3f97524b7e",
+      [
+       null,
+       {}
+      ]
      ]
     },
     "canvas-oversize-serialization.html": [

tests/wpt/mozilla/meta/mozilla/canvas/invalid.html.ini

@@ -0,0 +1,3 @@
+[invalid.html]
+  prefs: ["dom_canvas_backend:none"]
+