Set JEMalloc as the default allocator (instead of TCMalloc)...

Set JEMalloc as the default allocator (instead of TCMalloc)

I got this change approved to try out earlier, and landed it.  I rolled
it back (to be conservative) after I got a chromebot run started.  Since
that bot run didn't show any giant problems, this will re-land the
change, so we can see how users (such as on a dev channel release) do
with this allocator.

This is really experimental... but it will give us some info to make a
better decision between ongoing tuning of TCMalloc and at least 
temporarilly using JEMalloc.

tbr=mbelshe

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=27708