It seems like there is a memory leak if we haven't finished with the
marking phase of an incremental GC and SpiderMonkey has to trigger a
full GC because it runs out of memory. With this patch we stop trying to
make incremental GCs if we are above 1/2 of the runtime size and do Full
GCs instead. This should make such low memory conditions even less
likely than they were already after the previous patch. Also reduce the
maximum VFS cache size to 400 MB.
Refs #2808
This was SVN commit r15831.