On memory-constrained iOS devices, reduce the rate at which the JS heap grows before...

On memory-constrained iOS devices, reduce the rate at which the JS heap grows before a GC to try to keep more memory available for the system
https://bugs.webkit.org/show_bug.cgi?id=175041
<rdar://problem/33659370>

Reviewed by Filip Pizlo.

The testing I have done shows that this new function is a ~10%
progression running JetStream on 1GB iOS devices. I've also tried
this on a few > 1GB iOS devices, and the testing shows this is either neutral
or a regression. Right now, we'll just enable this for <= 1GB devices
since it's a win. In the future, we might want to either look into
tweaking these parameters or coming up with a new function for > 1GB
devices.

* heap/Heap.cpp:
* runtime/Options.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@220144 268f45cc-cd09-0410-ab3c-d52691b4dbfc