1571985 - Clean up Screen Orientation implementation

Status: NEW

(Core :: DOM: Core & HTML, task, P3)

Description

[Marcos Caceres [:marcosc]]

Please see:
https://bugzilla.mozilla.org/show_bug.cgi?id=1564621#c4

We are unsure as to the actual cause of the crash, but we suspect that when fullscreen exists,
the even listener is not being called. That was leaving mFullscreenListener alive when the DTOR gets called
hitting the MOZ_ASSERT(!mFullscreenListener).

We now always all call UnlockDeviceOrientation() in the DTOR to ensure that:

1. hal::UnlockScreenOrientation() gets called.
2. mFullscreenListener gets null'ed
3. we RemoveSystemEventListener() for "fullscreenchange".

However, we shouldn't need to do the above... "fullscreenchange" should have fired and called Unlock for us.