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
Review URL: http://codereview.chromium.org/256024

git-svn-id: http://src.chromium.org/svn/trunk/src@27708 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 120bab0caff9b0a009898ab4ef6797d32c11c6ef
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser