[sailfish-browser] Do not send "memory-pressure" to engine

Let gecko to do gc'ing when needed. Sending "memory-pressure" this
frequently (in common use cases) cause unnecessary page faults. In
addition, this looks to be leading to a higher memory consumption.
Merge pull request #217 from rainemak/avoidMemoryPressure

[sailfish-browser] Do not send "memory-pressure" to engine