Introduce flag --allocation-site-transitioning

Introduce flag --allocation-site-transitioning

We use allocation sites to optimize elements-kind transitioning.
For experimental purposes, it's useful to turn this on and off.

R=hpayer@chromium.org

[mvstanton, 2014-10-01 11:02:05]

Hi Hannes,
Not sure if we want to check this in or not, as it might be enough for
experiments to rebase on tip of tree. But this seems to be what is necessary to
turn off alloc. site transitioning. I ran kraken-once on this to get a 6.5%
degrade, roughly what was expected. Note that the flag
--allocation-site-transitioning has to be turned off for the snapshot as well as
any test runs.

[mvstanton, 2014-10-07 16:32:59]

Hi Hannes,
Here is the update we spoke of.

* Ran !transitioning && pretenuring -> tracing pretenuring stats had lots of
output in octane. Tracing transitioning stats (--trace-track-allocation-sites)
had ZERO output - as expected.
* Ran transitiong && !pretenuring -> not pretenuring stats output, but lots of
transitioning stats output - as expected.
thx,
--Michael