Displays array element with accessor descriptor more accurately

[nchevobbe]

In https://bugzilla.mozilla.org/show_bug.cgi?id=1358919 we will do some work on the backend side to send the appropriate information when elements of array have a getter.
On the Reps side, we should make sure to consume those data when they'll be available.

Steps to reproduce:
Evaluate x = [0]; Object.defineProperty(x, '0', { get: function() { return 0; } });

Expected results:
The rep displays as Array [ Getter ]

Actual results:
The rep displays as Array [ null ]

[nchevobbe]

Moved to firefox-devtools/devtools-core#396