Reintroduce runtime zone to Isolate.

Reintroduce runtime zone to Isolate.

In case tcmalloc is not being used, the malloc()/free() overhead
can be significant for several runtime functions like StringReplace.
Therefore we reintroduce the runtime_zone into Isolate and reenable
the segment caching functionality of Zone.

There's now also a simpler version of ZoneScope w/o nesting capabilities.

BUG=v8:2759
R=danno@chromium.org, yangguo@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=15465

[danno, 2013-07-03 11:36:38]

lgtm

[Yang, 2013-07-03 11:37:13]

On 2013/07/03 11:36:38, danno wrote:
> lgtm

lgtm

[Benedikt Meurer, 2013-07-03 11:40:37]

Committed patchset #1 manually as r15465 (presubmit successful).