Expose memory pressure notifications

I've been looking at implementing this, but linux doesn't have a supported API for applications to receive memory pressure notifications. It looks like WebKit did do something using memcg (https://bugs.webkit.org/show_bug.cgi?id=123532), but:

- This requires a 3.10 kernel, something that the phone doesn't have.
- It doesn't work on Ubuntu desktop either because applications can't open memory.pressure_level:

---------- 1 root root 0 Sep 23 12:41 /sys/fs/cgroup/memory/memory.pressure_level

I notice in https://code.google.com/p/chromium/codesearch#chromium/src/base/memory/shared_memory_android.cc that it uses Android's shared memory manager in the kernel rather than providing a memory pressure handler. Since we have this in our kernel (as far as I know) should we be using it?

FYI: WebKit developers are discussing alternative implementations not based on cgroups here https://bugs.webkit.org/show_bug.cgi?id=155255

> This requires a 3.10 kernel, something that the phone doesn't have.

 - Krillin (BQ Aquaris E4.5) has a 3.4.67 kernel
 - Arale (Meizu MX4) has a 3.10.35+ kernel
 - Frieza (BQ Aquaris M10) has a 3.10.93+ kernel

On those devices that have a kernel >= 3.10, it doesn’t look like memory cgroups are enabled though (I’m not seeing any /sys/fs/cgroup/memory entry).

The alternative approach pointed to by carloslp is basically regularly polling /proc/meminfo. We are already doing something similar in webbrowser-app, I wonder whether that mechanism should be moved to oxide?