Tweak memory usage in web-activity

[krstic]

Various tweaks for Gecko memory usage are present here: http://engtech.wordpress.com/2006/12/02/how-to-fix-the-firefox-memory-leak-firefox-hack/

We want to utilize these for the web activity. #4 isn't useful since we don't minimize windows, but having something like it -- perhaps simply on switching out of the activity -- would be good.

[marco]

I disabled links prefetch, the other values seem pretty decent to me (on a B3):

50 MB of disk cache 10 MB of mem cache 3 pages from the history in memory

If you feel any of these should be tweaked please reopen.