Bug 845147 - Set a magic pref in the reftests.
Marking more tests as needing the magic pref.
Moving some code in nsDocument.cpp.
Remove content.image.allow_locking pref.

We were not actually using this pref anywhere, even though we thought we
were on Android (the pref was only respected in content processes).  I
think the demonstrated confusion is reason enough to remove it.
WIP Bug 847221 - Separate out notion of "image is visible" and "image is in active tab".